Sahih al-Bukhari 6047
Sahih al-Bukhari · Book 78: Good Manners and Form (Al-Adab) · Calling bad names and cursing
Narrated Thabit bin Ad-Dahhak:
حَدَّثَنَا مُحَمَّدُ بْنُ بَشَّارٍ ، حَدَّثَنَا عُثْمَانُ بْنُ عُمَرَ ، حَدَّثَنَا عَلِيُّ بْنُ الْمُبَارَكِ ، عَنْ يَحْيَى بْنِ أَبِي كَثِيرٍ ، عَنْ أَبِي قِلاَبَةَ ، أَنَّ ثَابِتَ بْنَ الضَّحَّاكِ ، وَكَانَ، مِنْ أَصْحَابِ الشَّجَرَةِ حَدَّثَهُ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " مَنْ حَلَفَ عَلَى مِلَّةٍ غَيْرِ الإِسْلاَمِ فَهْوَ كَمَا قَالَ، وَلَيْسَ عَلَى ابْنِ آدَمَ نَذْرٌ فِيمَا لاَ يَمْلِكُ، وَمَنْ قَتَلَ نَفْسَهُ بِشَىْءٍ فِي الدُّنْيَا عُذِّبَ بِهِ يَوْمَ الْقِيَامَةِ، وَمَنْ لَعَنَ مُؤْمِنًا فَهْوَ كَقَتْلِهِ، وَمَنْ قَذَفَ مُؤْمِنًا بِكُفْرٍ فَهْوَ كَقَتْلِهِ " .
(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."