Let there arise out of you a group of people inviting to all that is good (Islâm), enjoining Al-Ma'rûf (i.e. Islâmic Monotheism and all that Islâm orders one to do) and forbidding Al-Munkar (polytheism and disbelief and all that Islâm has forbidden). And it is they who are the successful. (3:104).
Muslim relates on the authority of Abu Sa`id (radhiallahu `anhu) that the Prophet sallallahu `alayhi wa sallam said, "Whoever sees something evil should change it with his hand. If he cannot, then with his tongue; and if he cannot do even that, then in his heart. That is the weakest degree of faith."
In this lecture the speaker mentions examples from the time of the Companions in enjoing the good and forbidding the evil.