(8) باب قَوْلِ النَّبِيِّ صلى الله عليه وسلم " لاَ تَرْجِعُوا بَعْدِي كُفَّارًا، يَضْرِبُ بَعْضُكُمْ رِقَابَ بَعْضٍ
Narrated `Abdullah:
The Prophet, said, "Abusing a Muslim is Fusuq (evil doing) and killing him is Kufr (disbelief).
Reference: Sahih al-Bukhari 7076
In-book reference: Book 92, Hadith 27
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 9, Book 88, Hadith 197