What is the ruling on the prayer of a follower (ma'mum) if the follower suspects that the Imam does not pray on a continuous basis?

Whoever sometimes abandons prayer commits a grave sin and is considered a transgressor (fasiq). Muslims are not permitted to appoint him as an imam for their prayers; rather, he must be replaced by someone else as long as he remains in that state. If it is impossible to replace him, then one may pray behind him, provided he is not negligent concerning the rulings of purification, as prayer led by a transgressor is valid. All of this applies if they are certain of his abandonment of prayer. However, mere doubt does not permit entertaining ill thoughts about a Muslim without proof.