Narrated Al-Ahnaf bin Qais:

While I was going to help this man ('Ali Ibn Abi Talib), Abu Bakra met me and asked, "Where are you going?" I replied, "I am going to help that person." He said, "Go back for I have heard Allah's Messenger (s.a.w.a.) saying, 'When two Muslims fight (meet) each other with their swords, both the murderer as well as the murdered will go to the Hell-fire.' I said, 'O Allah's Messenger (s.a.w.a.)! It is all right for the murderer but what about the murdered one?' Allah's Messenger (s.a.w.a.) replied, "He surely had the intention to kill his companion."

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 (s.a.w.a.), viz. 'Ali. Thereupon he said to me: Ahnaf, go back, for I heard Allah's Messenger (s.a.w.a.) 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.