What is the ruling on the prayer of a congregant behind an imam who slaughters for other than Allah, and what must the congregant do to rectify the prayers in which he followed such an imam, in the event that it is impermissible?

If the mentioned Imam slaughters for other than Allah, intending to worship that (entity) instead of Allah, then he is considered a disbeliever, a disbeliever who is expelled from the religion, provided that he is not excused by ignorance, coercion, forgetfulness, or misinterpretation, and that the proof has been established against him by some of the people of knowledge.

In that case, the prayer of whoever prayed behind him becomes invalid, and they are obligated to repeat all the prayers they performed behind him. If his disbelief is manifest, such as a Jew or a Christian, then repeating the prayer is obligatory without dispute. But if his disbelief is concealed, such as an apostate or a zindeeq (heretic), then the correct view according to the majority is that repetition is obligatory.

If the aforementioned conditions or some of them are not met, then he is not judged as a disbeliever. However, it is preferable to pray behind someone else who is free from innovations.