Is it permissible for an Imam to lead prayers while being unjust, and to pray behind him?

It is permissible to pray behind a wicked Imam, although it is more befitting for someone else from among the people of knowledge, righteousness, and piety to lead the people in prayer. The proof for its permissibility is what Al-Bukhari narrated regarding Ibn Umar and Anas praying behind Al-Hajjaj, and what Muslim narrated regarding Abu Sa'id Al-Khudri praying behind Marwan.