This allegation is extremely old and is one made by the ignorant. Firstly in Promised Messiah(As)’s last writing he stated:
“This allegation which is leveled against me that I consider myself to be such a prophet who does not consider it necessary to follow the Holy Qur’an and have invented a new Kalimah, and new Qiblah and declare the Shari‘at or law of Islam to be abrogated, this allegation upon me is not true. Quite the contrary, I consider such a claim to be ‘Kufr’ or disbelief; and not from today, rather, as always, I have written in all of my books that I have not claimed any such prophethood; and this is an absolute accusation against me.”(Akhbar-e-ʻAm 26 May 1908)
The quote which the non Ahmadis try to use is from Arba’in #4 where Ahmad(As) is referring to the verse which a lot of us, Ahmadi Muslims quote. It is the verse of Surah al Haqqah where we show that a liar claiming to get revelations from Allah when he really hasn’t gotten any, would die within 23 years, so in reply to this Ahmad(As) states:
“And if you assert that only such a person is destroyed who falsely claims tobring a new law, and not every single person who forges [revelation of Allah]. Then firstly, this claim is without argument. Allah the Almighty has stipulated no such condition of ‘law’ in this forgery. In addition to this, oneshould also contemplate as to what [the true essence of] law really is? A person who presents various commandments and prohibitions through his revelation and establishes a system of order, is someone who is “lawbearing”.
Therefore, in light of this definition our opponents are also proven wrong, because there are commandments in my revelation as well. For example:
‘Say to the believing men that they should restrain their eyes and guard their private parts.
This [revelation] is written in Barahin-e-Ahmadiyyah, and it contains both acommand and a prohibition and 23 years have passed upon this revelation as well.(Arba‘in No. 4, Ruhani Khaza’in, Volume 17, pp. 435-436)
Ahmad(As) here shows that his opponents are incorrect and mistaken when they define law bearing. He is not claiming to be a prophet who has brought a religion at all. Ahmad(As) did not claim to bring anything new, for example the verse “say to the believing men that they strain their eyes and guard their private parts” is not something new rather its a verse of Quran as a reminder for the Muslims as Ahmad(As) was the reviver of the faith.
In Arba‘in, the Ahmad(As) stated that if those verses which we use to prove that liar would die within 23 years was only for law bearing, meaning a person whose revelations contain any command, then he too has received that type of revelation and should have been destroyed but yet nothing came in his way because Allah was helping him with each step he took.
Ahmad(as) has stated:
This eminence has been granted to me because of following the Holy Prophet may peace and blessings of Allah be upon him. Had I not been one of his followers and had I not been following his teachings faithfully, I could never have achieved this high status of communion with Allah, even though my good deeds had piled up to the height of the mountains. This is because all prophethoods have now come to an end. Now no law-bearing prophet can ever be raised, but a non-law-bearing prophet can still appear. But he must always be a follower of the Holy Prophet may peace and blessings of Allah be upon him. I am, therefore, a follower as well as a prophet. (Tajalliyti Ilahiyya, pg 19-20. Roohani Khazain, Vol 20, pg 411)
He also states:
“If it is asked that the Holy Prophet (saw) him being Khatamun Nabiyyeen, how can any other prophet arise after him? The answer is that without a doubt, no new or old prophet can come after the Holy Prophet as you believe that Jesus, being a prophet, would come in the latter days. Your doctrine is that for 40 years Jesus would continue to be a prophet and a recipient of divine revelation, a period that by far exceeds the period of the Prophethood of the Holy Prophet. Such a doctrine is, no doubt, sinful and its falsity is borne witness to by the Hadith: ‘There will be no prophet after me’.
I am utterly opposed to all such doctrines and I have firm belief in the verse:‘But he is the Messenger of Allah and Khatamun Nabiyyeen’ (33:41).
This verse contains a prophecy of which our opponents are not aware, and that prophecy is that after the Holy Prophet, may peace and blessings of Allah be upon him, all doors of prophecy have been closed and that it is not possible now that a Hindu or a Jew or a Christian or a merely formal Muslim, should be able to establish the applicability of the word prophet to himself.
All windows of Prophethood are now closed except the window of complete obedience to the Holy Prophet.Therefore, he who approaches God through this window is reflectively clothed with the same cloak of Prophethood which is the cloak of the Muhammadi Prophethood. The Prophethood of such a one is not apart and distinct from the Prophethood of the Holy Prophet, inasmuch as he does not claim it in his own right but receives everything from the fountain of the Holy Prophet, not for himself but for his glory….
Therefore the concept of Khatamun Nabiyyeen has not been contravened by my advent, but it would certainly be contravened by the advent of Jesus a second time.” ( Ek Ghalti Ka Izalah ‘A Misconception Removed’)