There is a lot of proof, main ones being he went to Hajj and his cloak was full of Quranic verses, along with the book he always carried around. There’s is much more proof then this for example him marrying twice and second marriage to a Muslim girl who’s dad was a scholar, now we all know a scholar dad would not give away his daughter to a non Muslim
The Janam Sakhi states:
“Nanak said, 0: Rukn-ud-Din, hear from me the true reply: the saying of the Lord is written in the Book. That person will go to hell who does not repeat the Kalima, who does not keep the thirty fasts, and does not say the five prayers, who eats what is not lawful for him. These shall receive the punishment and the fire of the bottomless pit shall be his abode (Janam Sakhi Bhai Bala, p. 193)
“The Quran is divided into thirty sections, proclaim thou, this Quran in the four corners of this world. Declare the glory of one name only for none other is an associate with me. Nanak proclaims the word of God that came to him, thou hast been granted the rank of Sheikh, so thou shouldst abolish the worship of gods and goddesses and the old Hindu idol – temples.” (Janam Sakhi Bhai Bala, p. 194)
A few Shaloks (verses) read:
I have repeated one Kalima, there is none other.
I have repeated one Kalima, there is none other. Those who repeat the Kalima and are not devoid of the faith, shall not be burned on fire. Repeat the Holy Kalima of the Prophet, it shall cleanse thee of all sins. By repeating the Kalima, the punishment of this world, as well as the next is averted. Who ever repeats the Kalima, how shall he be punished? the merit of repeating the Kalima is that a person is cleansed of his sins.
Guru Nanak also carried a prayer mat around with him and had two main friends one Muslim and one Hindu, but whenever one wanted to join his community he took them to the Muslim. He stayed mostly around Muslim people and practised what Muslims practice.
The janam sakhi also states: 0: Rukn-ud-Din, hear from me the true reply: the saying of the Lord is written in the Book. That person will go to hell who does not repeat the Kalima, who does not keep the thirty fasts, and does not say the five prayers, who eats what is not lawful for him. These shall receive the punishment and the fire of the bottomless pit shall be his abode. It is also reported that Baba Nanak kept fasts for a whole year at Mecca and put his fingers in his ears and gave the call to prayer. It is also related that Nanak recited the Khutba of the Prophet and became happy.
Here is a picture of the Cloak/Chola of Guru Nanak:
It is possible that he agreed with some things Hindus did also but never something which was against Islam.
Interesting Story in Regards to Mirza Ghulam Ahmad(as) and the incident of the chola taken from life of Ahmad(as) by A.R. Dard(ra). This passage is from pages 448-460
One should imagine that the brutal treatment of Ahmad’(as)s forefathers by the Sikh chieftains in the dark days of the Punjab would have left a sting behind. But not so with Ahmad(as). Swami Dayanand, the Founder of the Arya Samaj, had written offensively about Baba Nanak, the Founder of Sikhism, in his book Satyarath Parkash (Ajmer 1948, p. 356). He had declared Nanak to be a perfect ignoramus who posed as one learned in Sanskrit out of his vanity and conceit; Nanak was according to him, made out to be a saint by ignorant people only after his death. Being a champion of truth and fairness, Ahmad(as)as took up the cudgels against Dayanand to protect the honour of Nanak, whose followers had either not read the book or taken no notice of it. So he began to write a book and announced it by a leaflet issued on September 9th, 1895. Sikhs are found in the Punjab, United Provinces, Sind, Jammu and Kashmir. Their number is estimated at 3,000,000. Sikhism was founded by Baba Nanak, a Khatri by birth, who was born at Talvandi near Lahore in 1469 A. D., and after travelling throughout a great part of Southern Asia, died at Kartarpur in Jullundur in 1539. The tenth successor of Nanak, Guru Govind Singh, gave this movement a different turn. What had sprung into existence as a quietest sect of a purely religious nature became a military society and a national movement. The savagery of the Sikh rule in the Punjab is well-known. The very words Sikha Shahi stand for oppression and tyranny. The British had to wage two wars with the Sikhs before they could restore peace and tranquility to the Punjab. Like Muslims, the Sikhs are a martial race and they supplied a large number of recruits not only during the mutiny of 1857 but also in the great war of 1914- 1918.
Ahmad(as) produced evidence from Sikh scriptures of the fact that Guru Nanak, the Founder of Sikhism, was converted to Islam during the latter part of his life. He maintained that Nanak was a great Muslim saint and that it was their political conflict with the Mughals that drove his followers away from his original teachings. Ahmad(as)s work was entitled Sat Bachan. It was completed by the end of November 1895. In the first part of this book Ahmad(as) refuted all the objections that had been levelled against Nanak by Dayanand. Then he separated historical fact from legendary lore that had gathered round the name of Nanak in Sikh writings. Imaginary stories are related about Nanak’s visit to Mecca. It is said that the town of Mecca and Kaaba kept moving from one place to another to keep towards Nanak’s feet out of respect and that the Meccans spoke to Nanak in Punjabi. Again, it is written that Nanak met in Mecca Imam A‘zam who had in fact died about 700 years earlier. Similarly, it is said that Nanak met Sayyid Abdul Qadir Jilani (rh) in Baghdad although the latter had died 400 years before him. Qaroon and Baba Farid Shakar Ganj(rh) are also made to meet Nanak; the former was a contemporary of Moses(as) and the latter had died 200 years before Nanak. There are contradictions even in the Granth Sahib, which was compiled 200 years after Nanak. But Ahmad(as) contended that there could be no contradiction in Nanak’s words, it was really later writers who interpolated their own ideas into the Granth Sahib. The Encyclopaedia Britannica says that the Granth contains the compositions of many people. A most convincing piece of evidence which Ahmad(as) discovered and recorded in his book is the Chola Sahib. It is a cloak of cotton material which is held very sacred by the Sikhs. A kind of anniversary is held every year in honour of the Chola Sahib at Dera Baba Nanak a small village in the Gurdaspur district in the Punjab, and the Sikhs gather together there in great numbers.
This village is only a few miles from Qadian. It is believed by the Sikhs almost universally that this cloak was a direct gift from God to Nanak. It was sent down from heaven. Nanak used to wear it. It is written in the Sakhi Chola Sahib that it came to the possession of Angad, the first successor of Nanak, who wrapped it round his head at the time of his accession to the gaddi to seek blessings from it. This became a sacred precedence and the custom was kept up by the first five successors. During the time of Arjan Das, a tank was being dug up at Amritsar by the Sikhs. A devoted disciple, Tota Ram, was also digging the earth among the faithful. Arjan was pleased at the sight and asked Tota Ram if he wanted anything. He took advantage of the propitious moment and asked for the Chola Sahib which the Guru was wearing on his head. The Guru could not refuse the request and the Chola was handed over to the disciple. Later it passed into the hands of Kabuli Mall who was a descendent of Nanak. It is still treasured by the family at Dera Baba Nanak in a huge building raised by ‘Ajab Singh to the east of the village. Distinguished Sikhs, including rulers of States like Rajah Sahib Singh, Rajah Bhup Singh, the Maharaja of Patiala, have from time to time presented covers for this sacred relic of Nanak. It is now hidden under a heap of covers. Ahmadas sent the following persons to Dera Baba Nanak to look at the Chola Sahib: Mirza Ya‘qub Baigra, Munshi Tajuddinra, Kh. Kamal-ud-Din and Miyan Abdur Rahman. The custodians of the Chola received the deputation well. The deputation reported that they had seen written on the Chola verses from the Holy Quran. Ahmad(as) decided to see this sacred cloak of Nanak himself. So he went to Dera Baba Nanak on Monday, September 30th, 1895. He was accompanied by the following: Maulawi Nur-udDin(ra), Maulawi Muhammad Ahsan(ra), Munshi Ghulam Qadir Fasihra, Sh. Abdur Rahim(ra), Sayyid Muhammad Ismael(ra), Maulawi Abdul Karim(ra), Sh. Rahmatullah Gujrati(ra), Mirza Ayyub Baig(ra), Mir Nasir Nawab(ra) and Sh. Hamid Ali(ra).
The custodians of the Chola did not know what was written on it. It was unusual that anyone should be interested to see it in the original. So the keeper hesitated to uncover the sacred treasure. But the silver key and the insistent requests of the party opened everything. Rs. 14 was paid to the keeper in ones or twos by different friends when he laid it absolutely bare. The whole writing on it was then copied out. It openly declared that Islam was the only true religion and that Muhammad(sa) was the Messenger of the One God. There was nothing else on it. The Arya Samajists told the people that there was Sanskrit also written on the Chola and that the Vedas were also praised. So Ahmad(as) announced that he would give Rs. 3,000 to anyone who proved these stories. Janam Sakhis of Angad and Bala are the oldest Sikh writings, and were compiled in the time of Nanak. Chola Sakhi is the book which is in the hands of the custodians. All their authentic books definitely state that the Chola belongs to Nanak and that it was given to him by God Himself from heaven (See Janam Sakhi 107 Bhai Bala, Mufid-e-‘Am Press, Lahore, 1896). Varan Bhai Gurdas (printed at the Mustafa-ePress Lahore, 1947 Samat, page 12) says that Nanak went to Mecca dressed in blue clothes as a Muslim saint with a prayer mat in his hand. He called out the Adhan and sat in the Mosque for prayers. The researches of Ahmad(as) also led to the discovery of the fact that Nanak performed Chillas (Muslim form of spiritual purification) at various places. At Sirsa there is a small building which is known as ‘Chilla Bawa Nanak’. He had it built as an adjunct to the tomb of the Muslim Saint, Shah Abdush Shakur (rh). Dr. Muhammad Ismael Khan was sent to Sirsa to investigate and his detailed report confirmed Nanak’s association with the place (See Gur Khalsa by Giyan Singh). At Multan also Nanak peroformed a 40 day Chilla at the shrine of Shah Shams Tabriz.
Ahmad(as) deputed Niaz Baig(ra) to make full enquiries on the subject, and his letter dated September 28th, 1895, shows that he went thoroughly into the matter and it was established that Nanak had performed the Chilla at Multan. Sikhs visit the two places as their sacred places for the same reason. Dr. Trumpp, a German missionary, who translated the Granth on behalf of the Punjab Government in 1877, thinks it not probable that Nanak visited Mecca. But he does not appear to be right. His knowledge of Punjabi dialects was quite insufficient and, therefore, he could not have access to all the original sources of his information. Muslim mystics of the Chishti school of thought definitely say that Nanak went to Mecca on pilgrimage twice and that he also stayed there for two years. God commanded Nanak to perform the Hajj (see Janam Sakhi Bhai Bala, Mufid-e-‘Am Press, Lahore, 1897, p. 136. Ibid. Nawalkishore press, Lahore, 1890). It may be mentioned here that Mr. M. A. Macauliffe, author of The Sikh Religion says: ‘The doctrines of Sikhism as set forth in the Granth are that it prohibits idolatry, hypocrisy, class exclusiveness, the concremation of widows, the immurement of women, the use of wine or other intoxicants, tobacco smoking, slander, and pilgrimage to the sacred rivers and tanks of the Hindus.’ It is admitted that when Nanak died, Muslims and Hindus both claimed to conduct his funeral prayers. Muslims do not offer funeral prayers unless they are certain that the deceased was a Muslim. Therefore, this incident further confirms the fact that Nanak was a Muslim. Though Ahmad(as) had done a great service to the Sikhs by answering the objections levelled against their Founder by Swami Dayanand, yet some of them were displeased and charged Ahmad(as) with insulting them by calling Guru Nanak a Muslim saint.
Therefore Ahmad(as) thought it necessary to refute the allegation by declaring that he meant no slight but that he had only brought to light some fresh evidence which showed that Nanak was a Muslim at heart, which was rather a compliment to their Founder. He also reproduced a few pages of his book Sat Bachan as a specimen to show how the Christians also had attacked the Sikh religion and how he had defended the honour of its Founder. The leaflets were dated November 20th and 25th, 1895. Sardar Rajinder Singh published a pamphlet entitled Khabti Qadiani Ka ‘Ilaj in which he made baseless attacks upon the Holy Founder(sa) of Islam and refused to believe that Guru Nanak was a Muslim. In reply to this Ahmad(as) issued a leaflet on April 18th, 1897, in which he referred to historical evidence on the point; at the same time he declared that he had seen the Guru in a vision and was assured by him that he regarded Islam as the true religion. If, however, Rajinder Singh insisted upon what he had published, then he should come forward to declare the same in a public meeting under a most solemn oath, so that God might give His judgment in the matter. Rajinder Singh, however, was not prepared for this and nothing was heard again about the matter. Mr. Thomas Patrick Hughes, B. D., M. R. A. S. has published a Dictionary of Islam (W. H. Allen & Co., London, 1895) and it is a significant fact that he has included in this work an article on Sikhism. In its Preface (p. vi) it is said that: ‘Sikhism has been treated as a sect of Islam.’ The scholarly article has been written by Mr. Frederic Pincott, M. R. A. S. He says: “From these old books (the Janam Sakhis) we learn that, in early life, Nanak, although a Hindu by birth, came under Sufi influence, and was strangely attracted by the saintly demeanour of the faqirs who were thickly scattered over Northern India and swarmed in the Punjab (p. 583)… The traditions of Nanak preserved in the Janam Sakhi, are full of evidences of his alliance with Muhammadanism (p. 585)…
The ordinary Janam Sakhis current in the Panjab vary the account somewhat by saying that when the Khan reproved Nanak for not coming to him when sent for, the latter replied: ‘Hear, O Nawab, when I was thy servant I came before thee; now I am not thy servant; now I am become the servant of Khuda (God).’ The Nawab said: ‘Sir, (if) you have become such, then come with me and say prayers (NiwajNimaz, see Prayer). It is Friday.’ Nanak said: ‘Go, Sir.’ The Nawab, with the Qazi and Nanak, and a great concourse of people, went into the Jami Masjid and stood there. All the people who came into the Masjid began to say, ‘Today Nanak has entered this sect.’ There was a commotion among the respectable Hindus in Sultanpur; and Jairam, being much grieved, returned home. Nanaki, perceiving that her husband came home dejected, rose up and said, ‘Why is it that you are today so grieved’? Jairam replied, ‘Listen, O servant of Paramesur (God), what has thy brother Nanak done! He has gone, with the Nawab, into the Jami Masjid to pray; and, in the city, there is an outcry among the Hindus and Musalmans that Nanak has become a Turk (Muslim) today.’ (India Office MS, No. 2885, fol. 39). From the foregoing it is perfectly clear that the immediate successors of Nanak believed that he went very close to Muhammadanism; and we can scarcely doubt the accuracy of their view of the matter when we consider the almost contemporaneous character of the record from which extracts have been given, and the numerous confirmatory evidences contained in the religion itself… Nanak then proceeded to Panipat and was met by a certain Shaikh Tatihar, who accosted him with the Muhammadan greeting, “Peace be on thee, O Darvesh”, (Salam Aleka Darves); to which Nanak immediately replied, “And upon you be peace, O servant of the Pir” (aleka us salamu, ho Pir ke dastapes)” India Office MS, No. 1728, fol. 48 (Here we find Nanak both receiving and giving the Muhammadan salutation, and also the acknowledgment that he was recognized as a darvesh (p. 586)…
Nanak recommenced his missionary work, and is described as meeting a muhammadan named Miyan Mitha, who called upon him for the Kalimah or Muhammadan confession of faith (fol. 143); which leads to a long conversation, in which Nanak lays emphasis on the Sufi doctrine of the Unity of God. In this conversation Nanak is made to say, “The book of the Quran should be practised” (fol. 144). He also acknowledged that “Justice is the Quran” (fol. 148). When the Miyan asked him what is the one great name, Nanak took him aside and whispered in his ear, “Allah” (God)… In the course of his teaching in Makkah, Nanak is made to say: “Though men, they are like women, who do not obey the Sunnat, and Divine commandement, nor the order of the book (i.e., the Quran)” (I.O.MS No. 1728, fol. 212). He also admitted the intercession of Muhammad, denounced the drinking of bhang, wine, etc., achnowledged the existence of hell, the punishment of the wicked, and the resurrection of mankind; in fact, the words here ascribed to Nanak contain a full confession of Islam. These tenets are, of course, due to the narrator of the tale; and are only useful as showing how far Nanak’s followers thought it possible for him to go (p. 588)… The early Gurus openly assumed the manners and dress of faqirs, thus plainly announcing their connection with the Sufistic side of Muhammadanism. In pictures they are represented with small rosaries in their hands, quite in Muhammadan fashion, as though ready to peform Zikr. Guru Arjan, who was the fifth in succession from Nanak, was the first to lay aside the dress of a faqir (p. 584)… The circumstances which led to the entire reversal of the project, and produced between Muhammadans and Sikhs the deadliest of feuds, does not come within the purview of the present article. It is enough to state that the process was gradual, and was as much due to political causes as to a steady departure from the teachings of the founder of Sikhism’ (p. 591). In the face of all these facts it would not be easy to contend that Nanak was not a Muslim or that ‘the religion of Nanak was really intended as a compromise between Hinduism and Muhammadanism.’
He believed in the Unity of God and subscribed to the whole of the Islamic creed which he preached wherever he went. He offered the Muslim prayers and carried out practically all the other ordinances of Islam. He associated with Muslims, he greeted with the Muslim greeting and clothed himself with the Muslim dress. What else was necessary for him to become a Muslim? He was no doubt a Muslim of the first order; in fact, a saint and a holy personage. He combined in himself the best of all religions; because, as a Muslim, he walked in the footsteps of all the true Prophets of God. But he made no compromise. On the other hand he openly exposed the errors of the people of his time and exhorted all, by precept and example, to live pure and righteous lives. (Pages 448-460 Life of Ahmad, by A.R. Dard(ra))