Q : Is it permissible to offer Salah behind a man believing in a doctrine different from Ahl Al Sunnah walJama`ah such as AlAsh'ari (a follower of Asha`irah i.e. a Muslim group that bases its creedal issues on logic), for example?
A : The more preponderant opinion and Allah knows best is that it is permissible to offer prayer behind any Muslim while it is impermissible to do so behind nonMuslims. This is the opinion of a group of scholars and it is the correct opinion. As for a person who says that it is impermissible to offer prayer behind a sinner, such an opinion is not the best since the Prophet (peace be upon him) permitted offering salah behind rulers. Yet, many among them are wrongdoers. Ibn 'Umar, Anas, and a group of Muslim scholars offered prayer behind AlHajjaj who was one of the worst wrongdoers.
In conclusion, Salah is possible behind a mubtadi` (one who introduces innovations in religion) whose Bid`ah (innovation in religion) does not render him a nonMuslim. It is possible to be offered behind a fasiq (someone flagrantly violating Islamic law) who commits manifest sins that do not render him a nonMuslim. Yet, when offering prayer in congregation, Ahlul Sunnah walJama`ah (those adhering to the Sunnah and the Muslim community) should give priority to the best among them.