From Sahih Bukhari (Hadith 6045):
Narrated Abu Dhar:That he heard the Prophet (ﷺ) 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
Reference: Bukhari 6045