Sunan an-Nasa'i 4123
سنن النسائي#4123Sahihأَخْبَرَنَا أَحْمَدُ بْنُ عَبْدَةَ، عَنْ حَمَّادٍ، عَنْ أَيُّوبَ، وَيُونُسَ، وَالْعَلاَءِ بْنِ زِيَادٍ، عَنِ الْحَسَنِ، عَنِ الأَحْنَفِ بْنِ قَيْسٍ، عَنْ أَبِي بَكْرَةَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " إِذَا الْتَقَى الْمُسْلِمَانِ بِسَيْفَيْهِمَا فَقَتَلَ أَحَدُهُمَا صَاحِبَهُ فَالْقَاتِلُ وَالْمَقْتُولُ فِي النَّارِ " .
It was narrated that Abu Bakrah said:"The Messenger of Allah [SAW] said: 'If two Muslims fight with swords, and one of them kills the other, then the killer and the slain will both be in Hell
Scholarly Authentication
The label above is the grade most of these scholars gave — one dissenting opinion no longer speaks for all of them. If any scholar calls a narration fabricated, that verdict is shown instead, whatever the others say. Each scholar's own wording:
In-book reference: 37:158