The ruling on praying behind one who opposes the people of the Sunnah, such as an Ash'ari or the like

Q: Is it permissible to pray behind a person whose belief contradicts 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 anyone whom we rule to be a Muslim is valid to pray behind, and whoever is not, is 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 an innovator who commits an innovation that does not take him out of Islam, or an open sinner whose open sinfulness does not take him out of Islam. However, one should appoint a follower of the Sunnah as the imam. Likewise, if a group is gathered in a place, they should put forward their best one.