Is it permissible to withdraw from the prayer if it becomes apparent that the one who stepped forward to lead the prayer is a transgressor (fasiq) or known for bad manners?

Praying behind an open transgressor (fasiq) is disliked by the jurists, though it is considered valid. The Companions prayed behind imams known for their transgression, such as Ibn Umar praying behind Al-Hajjaj. It is permissible for a Muslim to pray behind someone described in the question, especially if no one better is available. It is not permissible to interrupt an obligatory prayer after having started it for this reason, because it is not permissible to interrupt an obligatory act of worship after commencing it without a legal justification, in accordance with the Almighty’s saying: "And do not invalidate your deeds."