Praying behind an Imam who has fallen into Bid'ah
Q. Can one pray behind an Imam who has fallen into Bid'ah?
A. Innovations are not all of the same degree. Some of them have to do with beliefs, some have to do with minor practical issues of sharee‘ah; some put a person beyond the pale of Islam whereas others do not. In some cases the daa‘iyah may think that something is an innovation when it is not, or there may be a scholarly opinion which is less correct than the opinion with which he is familiar, or there may be another opinion on the matter of which he is not aware.
Usually questions are asked about an imam who is doing innovated actions and the ruling thereon, and the answers are well known.
If his bid‘ah is one that constitutes kufr, then it is not permissible to pray behind him; if it does not constitute kufr, then it is valid to pray behind him, but it is better to pray behind an imam who is of Ahl al-Sunnah.
Concerning such cases al-Hasan al-Basri (may Allaah have mercy on him) said: ’Pray, and he is accountable for his bid‘ah".