Sahih al-Bukhari Book 78, Hadith 75
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."
Allah's Messenger (ﷺ) said, "Abusing a Muslim is Fusuq (i.e., an evil-doing), and killing him is Kufr (disbelief).
Allah's Messenger (ﷺ) was neither a Fahish (one who had a bad tongue) nor a Sabbaba (one who abuses others) and he used to say while admonishing somebody, "What is wrong with him? May dust be on his forehead!"