Sahih Muslim Book 54, Hadith 19
When two Muslims confront each other with their swords, both the slayer and the slain are doomed to Hell-Fire.
I set out with the intention of helping this person (Hadrat 'Ali) when Abu Bakra met me. He said: Ahnaf, where do you intend to go? I said: I intend to help the cousin of Allah's Messenger (ﷺ), viz. 'Ali. Thereupon he said to me: Ahnaf, go back, for I heard Allah's Messenger (ﷺ) as saying: When two Muslims confront one another with swords (in hand) both the slayer and the slain would be in Fire. He (Ahnaf) said: I said, or it was said: Allah's Messenger, it may be the case of one who kills. but what about the slain (why he would be put in Hell-Fire)? Thereupon he said: He also intended to kill his companion.
This hadith has been narrated on the authority of Hammad through another chain of transmitters.