Musnad Ahmad 1156:256

'Affan narrated to us, Hammam narrated to us, Qatadah narrated to us, from al-Hasan, from Dabbah ibn Mihsan al-'Anazi, from Umm Salamah, from the Prophet (peace and blessings of Allah be upon him), who said: "There will be commanders, and you will recognize [some of their actions] and disapprove [of others]. Whoever recognizes [the evil] is absolved, and whoever disapproves [of it] is safe; but [the sin falls upon] whoever is pleased and follows." They said: "O Messenger of Allah, shall we not kill their wicked ones?" He said: "No, as long as they pray."