Sahih Muslim 49 a
The Book of Faith
Clarifying that forbidding evil is part of faith, faith increases and decreases; Enjoying what is good and forbidding what is evil are obligatory
It is narrated on the authority of Tariq b. Shihab: It was Marwan who initiated (the practice) of delivering khutbah (address) before the prayer on the 'Id day. A man stood up and said: Prayer should precede khutbah. He (Marwan) remarked, This (practice) has been done away with. Upon this Abu Sa'id remarked: This man has performed (his duty) laid on him. I heard the Messenger of Allah as saying: He who amongst you sees something abominable should modify it with the help of his hand; and if he has not strength enough to do it, then he should do it with his tongue, and if he has not strength enough to do it, (even) then he should (abhor it) from his heart, and that is the least of faith.