Sunan an-Nasa'i, Volume 5, Book 37, Hadith 4127 (DarusSalam) ⓘ

It was narrated that Abu Bakrah said: "I heard the Messenger of Allah [SAW] say: 'If two Muslims confront each other with their swords and one of them kills the other, both the killer and the slain will be in Hell.'" They said: "O Messenger of Allah, (we understand about) the killer, but what about the one who is killed?" He said: "He wanted to kill his companion."

أَخْبَرَنَا أَحْمَدُ بْنُ فَضَالَةَ، قَالَ حَدَّثَنَا عَبْدُ الرَّزَّاقِ، قَالَ أَنْبَأَنَا مَعْمَرٌ، عَنْ أَيُّوبَ، عَنِ الْحَسَنِ، عَنِ الأَحْنَفِ بْنِ قَيْسٍ، عَنْ أَبِي بَكْرَةَ، قَالَ سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ ‏"‏ إِذَا تَوَاجَهَ الْمُسْلِمَانِ بِسَيْفَيْهِمَا فَقَتَلَ أَحَدُهُمَا صَاحِبَهُ فَالْقَاتِلُ وَالْمَقْتُولُ فِي النَّارِ ‏"‏ ‏.‏ قَالُوا يَا رَسُولَ اللَّهِ هَذَا الْقَاتِلُ فَمَا بَالُ الْمَقْتُولِ قَالَ ‏"‏ إِنَّهُ أَرَادَ قَتْلَ صَاحِبِهِ ‏"‏ ‏.‏