(44) باب مَا يُنْهَى مِنَ السِّبَابِ وَاللَّعْنِ
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: Sahih al-Bukhari 6045
In-book reference: Book 78, Hadith 75
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 8, Book 73, Hadith 71