Sunan Ibn Majah, Hadith 3963 ⓘ
It was narrated from Anas bin Malik that the Prophet said: “There are no two Muslims who confront one another with their swords, but both the killer and the slain will be in Hell.”
حَدَّثَنَا سُوَيْدُ بْنُ سَعِيدٍ، حَدَّثَنَا مُبَارَكُ بْنُ سُحَيْمٍ، عَنْ عَبْدِ الْعَزِيزِ بْنِ صُهَيْبٍ، عَنْ أَنَسِ بْنِ مَالِكٍ، عَنِ النَّبِيِّ ـ صلى الله عليه وسلم ـ قَالَ " مَا مِنْ مُسْلِمَيْنِ الْتَقَيَا بِأَسْيَافِهِمَا إِلاَّ كَانَ الْقَاتِلُ وَالْمَقْتُولُ فِي النَّارِ " .
also referenced under: Book 36, Hadith 38 (In-Book) & Volume 5, Book 36, Hadith 3963 (DarusSalam)