Sahih Bukhari Volume 008, Book 073, Hadith Number 071.

Narated By Abu Dhar : That he heard the Prophet saying, "If somebody accuses another of Fusuq (by calling him 'Fasiq' i.e. a wicked person) or accuses him of Kufr, such an accusation will revert to him (i.e. the accuser) if his companion (the accused) is innocent."