Al-Adab Al-Mufrad, Hadith 431 ⓘ
'Abdullah reported that the Prophet, may Allah bless him and grant him peace, said, "Reviling a Muslim is deviant behaviour and killing him is disbelief."
حَدَّثَنَا سُلَيْمَانُ بْنُ حَرْبٍ، قَالَ: حَدَّثَنَا شُعْبَةُ، عَنْ زُبَيْدٍ قَالَ: سَمِعْتُ أَبَا وَائِلٍ، عَنْ عَبْدِ اللهِ، عَنِ النَّبِيِّ صلى الله عليه وسلم: سِبَابُ الْمُسْلِمِ فُسُوقٌ، وَقِتَالُهُ كُفْرٌ.
also referenced under: Book 24, Hadith 13 (In-Book) & Book 24, Hadith 431 (English)