Is the prayer valid behind a grave-worshipping imam on whom the proof has not been established?

Question:
May Allah reward you. The questioner asks: Is the prayer valid behind a grave-worshipping imam on whom the proof has not been established?

Answer:
The proof, my brother, has been established since Allah sent His Messenger (peace and blessings be upon him). The proof is established, especially for those who claim Islam. The proof is established against them, because they read the Qur'an.
And they read the hadiths and claim affiliation with Islam. Islam forbids the worship of graves. The Qur'an forbids shirk (associating partners with Allah) in all its forms. The Sunnah of the Messenger (peace and blessings be upon him) forbids building over graves, forbids supplication at graves, and forbids prayer at graves. This is clear in Islam and not hidden, especially from one who claims to be a Muslim. Perhaps he is among the scholars, perhaps he is a scholar, but he is among the scholars of misguidance.
The proof has been established since Allah sent His Messenger (peace and blessings be upon him). Allah the Exalted said: Allah has sent the Messenger (peace and blessings be upon him) and made clear to the people, and established the proof against all people. Yes, so how can he claim to be a Muslim while he worships graves? Glory be to Allah. Islam is in one direction, and he is in another direction, far from Islam.