Sunan an-Nasa'i 4123
سنن النسائي#4123SahihIt 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
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahSahih
Al-AlbaniSahih
Zubair Ali ZaiHasan
In-book reference: 37:158