What is the ruling on praying behind an Imam who is an enemy of Ahl al-Sunnah wal-Jama'ah?

No one bears enmity towards the people of Sunnah because of the Sunnah except an innovator (mubtadi'), and prayer behind an imam whose innovation is not considered disbelief is valid. However, if another imam is available, he is preferable. But if his innovation is considered disbelief, then prayer behind him is not valid.