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 ".
• Sahih al-Bukhari, 7076
• Sahih al-Bukhari, Vol. 9, Book of Trials & Tribulations, Hadith 197
• Sahih al-Bukhari, Book of Trials & Tribulations, Hadith 197