One of the great prophecies of Hazrat Mirza Ghulam Ahmadas was that Allah had shown him that Guru Nanak was a Muslim. Ahmadas stated:
” On one occasion I saw Bava Nanak in a dream in which he declared himself a Muslim. I also saw a Hindu drinking from his fountain and I said to the Hindu: The water of this fountain is not clear, drink from our fountain. This was thirty years ago and I related my dream to several Hindus and I was certain that confirmation of this would become available in due course. Accordingly, after some years, this dream was fulfilled very clearly. Three hundred years after the death of Bava Nanak, we found access to his robe which proves clearly that he was a Muslim. This robe, which is a kind of outer covering, is preserved with great reverence as a relic at Dera Baba Nanak [District Gurdaspur] in the custody of his descendants” (Nuzul-ul-Masih, Pages 203-204, Ruhani Khazain, Volume 18, Page 581-582)
Further explaining this vision he stated:
” The declaration by Bava Nanak in my dream that he was a Muslim meant that sometime his being a Muslim will become public. It is for this purpose that I wrote my book ‘Satt Bachan’. My saying to the Hindu: ‘The water of this fountain is not clear, drink from our fountain’; meant that a time will come when the truth of Islam will become manifest to the Hindus and the Sikhs and the fountain of Bava, which the Sikhs have foolishly rendered muddy, will be cleared up through me.”(Nuzulul-Masih, Page 205, Ruhani Khazain, Volume 18, Page 583)
” It should be remembered that I have seen Bava Nanak in my visions twice and that he confessed that he had obtained illumination from the same light [of Islam]. I consider uttering of nonsense and falsehood like devouring carrion; I affirm only that which I have seen. That is why I hold Bava Nanak in honour for I know that he drank from the same fountain from which we drink; and God knows that I talk out of the knowledge that He has bestowed upon me.” (Announcement of April 18, 1897, Majumu’a Ishtiharat, Volume 2, Pages 396-397)
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:
If one wants to learn more about this prophecy they can refer to Life of Ahmad Pages 448-460)