Hadith

​An Nasai (The Book of Fighting [The Prohibition of Bloodshed] – 157)
It was narrated that Abu Bakar 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.” (Sahih)