What is the ruling regarding a preacher who justifies praying in mosques built upon graves, and is it permissible to pray behind him?

Prayer in mosques built over graves is strictly forbidden, as the Prophet, peace and blessings be upon him, cursed the Jews and Christians for taking the graves of their prophets as places of worship. As for the noble Prophetic Mosque, it was not built over a grave. Rather, the Prophet, peace and blessings be upon him, was buried in Aishah's chamber, which was later incorporated into the mosque with its door sealed off to prevent the grave from being taken as a place of celebration. Prayer behind an innovator is valid as long as their innovation does not reach the level of disbelief.