Ruling on the denial of a Muslim's Iman

A: This is not permissible, in fact it is Haram (prohibited), because of what was narrated on the authority of Ibn `Umar (may Allah be pleased with them both) who reported that the Messenger of Allah (peace be upon him) said, If a man says to his brother (in Islam), "O you Kafir (disbeliever)!" one of them will deserve the title. Either the person addressed is so or, if not, it (the title of Kafir) will revert to him (the speaker). Agreed upon by Al-Bukhary and Muslim. It also was reported on the authority of Abu Dhar (may Allah be pleased with him) that he heard the Messenger of Allah (peace be upon him) (Part No. 2; Page No. 138) saying, Anyone who accuses a person of Kufr (disbelief) or says, “O enemy of Allah,” and they are not so, it (the accusation) will revert to them (the accuser). Agreed upon by Al-Bukhary and Muslim. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.