Prohibition of Calling a Muslim an Infidel

Imam Al-Nawawi’s Riyad-us-Saliheen
Chapter 326
Prohibition of Calling a Muslim an Infidel

1732. Ibn `Umar (May Allah be pleased with them) said: The Messenger of Allah (sallallaahu ’alayhi wa sallam) said, “When a person calls his brother (in Islam) a disbeliever, one of them will certainly deserve the title. If the addressee is so as he has asserted, the disbelief of the man is confirmed, but if it is untrue, then it will revert to him.”
[Al-Bukhari and Muslim].

1733. Abu Dharr (May Allah be pleased with him) said: I heard the Messenger of Allah (sallallaahu ’alayhi wa sallam) saying, “If somebody accuses another of disbelief or calls him the enemy of Allah, such an accusation will revert to him (the accuser) if the accused is innocent.
[Al-Bukhari and Muslim].