Ahnaf b. Qais reported: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

Ahnaf b. Qais reported on the authority of Abu Bakra that Allah's Messenger (ﷺ) said:When two Muslims confront each other with their swords, both the slayer and the slain are doomed to Hell-Fire

This hadith has been narrated on the authority of Hammad through another chain of transmitters

Abu Bakra reported Allah's Messenger (ﷺ) as saying:When two Muslims (confront each other) and the one amongst them attacks his brother with a weapon, both of them are at the brink of Hell-Fire. And when one of them kills his companion, both of them get into Hell-Fire