Correcting People's Mistakes

The Messenger of Allaah (sallallaahu 'alayhi wa sallam) said:

"Whomsoever from amongst you see an evil he should change it with his hand. If he does not have the power to do this, then he should change it with his tongue. If he does not have the ability to do even this, then he should change it with his heart

(by scheming in the eradication of this evil)."

(Hadith)