The ruling on praying behind one whose creed differs from that of the people of the Sunnah

Q: Is it permissible to pray behind one whose creed differs from that of the people of the Sunnah and the community, such as an Ash'ari, for example?

A: The most likely view, and Allah knows best, is that it is valid to pray behind anyone whom we rule to be a Muslim, and it is not valid to pray behind one whom we do not. This is the opinion of a group of scholars, and it is the most correct.
As for the one who says that the prayer is not valid behind a sinner, his statement is the weaker position. The evidence for this is that the Prophet (peace and blessings be upon him) granted a dispensation for praying behind the rulers, and many of the rulers were sinners. Ibn 'Umar, Anas, and a group of others prayed behind al-Hajjaj, who was among the most unjust of people.
The conclusion is that the prayer is valid behind one who commits a religious innovation that does not take him out of Islam, or one who commits open sinfulness that does not take him out of Islam.
However, one should appoint a follower of the Sunnah. Likewise, if a group is gathered in a place, they should put forward their best one.