Sahih al-Bukhari 7076
Sahih al-Bukhari · Book 92: Afflictions and the End of the World · “Do not renegade as disbelievers after me by striking the neck of one another.”
Narrated `Abdullah:
حَدَّثَنَا عُمَرُ بْنُ حَفْصٍ ، حَدَّثَنِي أَبِي ، حَدَّثَنَا الأَعْمَشُ ، حَدَّثَنَا شَقِيقٌ ، قَالَ قَالَ عَبْدُ اللَّهِ قَالَ النَّبِيُّ صلى الله عليه وسلم " سِبَابُ الْمُسْلِمِ فُسُوقٌ، وَقِتَالُهُ كُفْرٌ " .
The Prophet, said, "Abusing a Muslim is Fusuq (evil doing) and killing him is Kufr (disbelief).