Chapter: Whoever calls his brother a Kafir without any grounds
Narrated Thabit bin Ad-Dahhak:The Prophet (ﷺ) said, "Whoever swears by a religion other than Islam (i.e. if he swears by saying that he is a non-Muslim in case he is telling a lie), then he is as he says if his oath is false and whoever commits suicide with something, will be punished with the same thing in the (Hell) fire, and cursing a believer is like murdering him, and whoever accuses a believer of disbelief, then it is as if he had killed him."
Reference
Sahih al-Bukhari 6105
In-book reference
Book 78, Hadith 132
USC-MSA web (English)
Vol. 8, Book 73, Hadith 126
Translation by Dr. M. Muhsin Khan · Language: EN
Sahih
Arabic Text
حَدَّثَنَا مُوسَى بْنُ إِسْمَاعِيلَ، حَدَّثَنَا وُهَيْبٌ، حَدَّثَنَا أَيُّوبُ، عَنْ أَبِي قِلاَبَةَ، عَنْ ثَابِتِ بْنِ الضَّحَّاكِ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " مَنْ حَلَفَ بِمِلَّةٍ غَيْرِ الإِسْلاَمِ كَاذِبًا فَهْوَ كَمَا قَالَ، وَمَنْ قَتَلَ نَفْسَهُ بِشَىْءٍ عُذِّبَ بِهِ فِي نَارِ جَهَنَّمَ، وَلَعْنُ الْمُؤْمِنِ كَقَتْلِهِ، وَمَنْ رَمَى مُؤْمِنًا بِكُفْرٍ فَهْوَ كَقَتْلِهِ ".