Sunan an-Nasa'i 4119
سنن النسائي#4119SahihIt 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
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahSahih
Al-AlbaniSahih
Zubair Ali ZaiSahih
In-book reference: 37:154