It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "When a person calls another a disbeliever (kafir), then one of them becomes a disbeliever. If the one who was called a disbeliever is truly (because of his sin) a disbeliever, then he (the accuser) has spoken the truth. But if he is not as he was described, then the burden of that statement will fall upon the one who said it, and he will become a disbeliever."