Sahih al-Bukhari, Volume 9, Book 83, Hadith 7 (USC-MSA) ⓘ
Narrated `Abdullah bin `Umar: The Prophet said, "After me (i.e. after my death), do not become disbelievers, by striking (cutting) the necks of one another.
حَدَّثَنَا أَبُو الْوَلِيدِ، حَدَّثَنَا شُعْبَةُ، قَالَ وَاقِدُ بْنُ عَبْدِ اللَّهِ أَخْبَرَنِي عَنْ أَبِيهِ، سَمِعَ عَبْدَ اللَّهِ بْنَ عُمَرَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " لاَ تَرْجِعُوا بَعْدِي كُفَّارًا يَضْرِبُ بَعْضُكُمْ رِقَابَ بَعْضٍ " .
also referenced under: Book 87, Hadith 7 (In-Book) & Hadith 6868 (DarusSalam)