Sahih al-Bukhari Book 78, Hadith 76
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!"
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."
(who was one of the companions who gave the pledge of allegiance to the Prophet (ﷺ) underneath the tree (Al-Hudaibiya)) Allah's Messenger (ﷺ) said, "Whoever swears by a religion other than Islam (i.e. if somebody swears by saying that he is a non-Muslim e.g., a Jew or a Christian, etc.) in case he is telling a lie, he is really so if his oath is false, and a person is not bound to fulfill a vow about a thing which he does not possess. And if somebody commits suicide with anything in this world, he will be tortured with that very thing on the Day of Resurrection; And if somebody curses a believer, then his sin will be as if he murdered him; And whoever accuses a believer of Kufr (disbelief), then it is as if he killed him."