Sahih al-Bukhari, Hadith 7076 ⓘ
Narrated `Abdullah: The Prophet, said, "Abusing a Muslim is Fusuq (evil doing) and killing him is Kufr (disbelief).
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 " .
also referenced under: Book 92, Hadith 27 (In-Book) & Volume 9, Book 88, Hadith 197 (USC-MSA)