Riyad Us-Saliheen (Gardens of the Righteous)

Chapter 326
Prohibition of Calling a Muslim an Infidel

1732. Ibn `Umar (May Allah be pleased with them) said: The Messenger of Allah (PBUH) 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].

Commentary:  We learn from this Hadith that to call a Muslim a Kafir is strictly prohibited because if the one who has been called as such does not have the traits which makes him a Kafir, then the one who says it will be held Kafir.

1733. Abu Dharr (May Allah be pleased with him) said: I heard the Messenger of Allah (PBUH) 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].

Commentary:  This Hadith also tells us that to call without reason any Muslim a Kafir or enemy of Allah is strictly prohibited.