From the route of Abu Sa^id al-Khudriyy, may Allah accept his deeds, who said: I heard the messenger of Allah, sallallahu ^alayhi wa sallam, say:
<<The one of you who sees an unlawful matter is obligated to change it with his hands, and if unable then with his tongue, and if unable then he is obligated to denounce it in his heart, which is the least that he can do.>>
Narrated by Muslim.