Can he pray behind someone who does not believe that Muhammad was the last of the prophets?
Question

I took part with some Muslims in a number of debates here in America about the last of the prophets (khaatam an-nabiyyeen). I hope that you can explain the following: What is meant by khaatam in Arabic; is it “last” or “seal”? – The evidence for that from the Qur’an and Sunnah – does the descent of ‘Eesa (peace be upon him) contradict the idea of the “khaatam an-nabiyyeen” or not? – What is the ruling on someone who does not believe that Muhammad (blessings and peace of Allah be upon him) was the last of the prophets or that there will be no prophet after him; is he a disbeliever or not? Is it permissible to pray behind him if there is no mosque in our neighbourhood?
Praise be to Allah.Prophet Muhammad (blessings and peace of Allah be upon him) is the last of the prophets. He (blessings and peace of Allah be upon him) said: “I have been favoured over the other prophets in five ways: I have been granted intercession, and the series of prophets ends with me…” Thus he (blessings and peace of Allah be upon him) stated that he is the last of them. Allah, may He be exalted, says: “but [he is] the Messenger of Allah and seal [i.e., last] of the prophets” [al-Ahzaab 33:40]. The word translated here as seal is khaatam; according to an alternative recitation, the phrase is khaatim an-nabiyyeena, meaning that he is the last of them. And the Prophet (blessings and peace of Allah be upon him) said: “I am the last of the prophets; there will be no prophet after me.” In other words, the series of Allah’s prophets ended with him, and there will be no prophet or messenger after him.
Undoubtedly when ‘Eesa (blessings and peace of Allah be upon him) comes down, he will follow the teachings of Muhammad (blessings and peace of Allah be upon him) and will rule in accordance with them. So he is not to be regarded as a prophet after the Messenger of Allah; rather he will be one of his ummah and will rule in accordance with his teachings (sharee‘ah).
Whoever claims that there will be a new prophet is a disbeliever. The same applies to one who says that it is possible that another prophet could come after Muhammad (blessings and peace of Allah be upon him), or that this law (sharee‘ah) is not the last law. Such a person is a disbeliever, and you should not pray behind him if it is known that this is his belief. If that is proven, then you have to repeat the prayer that you prayed behind him, even if you offer that prayer on your own.