Sunan an-Nasa'i 4119
سنن النسائي#4119Sahihأَخْبَرَنِي مُحَمَّدُ بْنُ إِسْمَاعِيلَ بْنِ إِبْرَاهِيمَ، قَالَ حَدَّثَنَا يَزِيدُ، - وَهُوَ ابْنُ هَارُونَ - قَالَ أَنْبَأَنَا سَعِيدٌ، عَنْ قَتَادَةَ، عَنِ الْحَسَنِ، عَنْ أَبِي مُوسَى الأَشْعَرِيِّ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " إِذَا تَوَاجَهَ الْمُسْلِمَانِ بِسَيْفَيْهِمَا فَقَتَلَ أَحَدُهُمَا صَاحِبَهُ فَهُمَا فِي النَّارِ مِثْلَهُ سَوَاءً " .
It was narrated from Abu Musa Al-Ash'ari that:The Prophet [SAW] said: "If two Muslims confront each other with swords and one of them kills the other, they 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: 37:154