Is it permissible to pray behind a sorcerer and charlatan imam, even if he recites the Qur’an proficiently?

It is not permissible to pray behind a sorcerer who claims to know the unseen or practices magic, because by doing so he commits an act of disbelief (kufr). As for an imam who has sins short of shirk (polytheism), prayer behind him is valid, but it is better to seek an imam who is more righteous and upright. Prayer is not valid behind one who invokes other than Allah or seeks help from the dead, because by this action he becomes one of the disbelievers.