Sahih al-Bukhari Book 78, Hadith 77
(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."
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!"
A man from the companions of the Prophet (ﷺ) said, "Two men abused each other in front of the Prophet (ﷺ) and one of them became angry and his anger became so intense that his face became swollen and changed. The Prophet (ﷺ) said, "I know a word the saying of which will cause him to relax if he does say it." Then a man went to him and informed him of the statement of the Prophet (ﷺ) and said, "Seek refuge with Allah from Satan." On that, angry man said, 'Do you find anything wrong with me? Am I insane? Go away!"