Volume 1, Book 2, Number 30:Bukhari Shareef
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 Apostle saying, 'When two believers 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 Apostle! It is all right for the murderer but what about the murdered one?' Allah's Apostle replied, "He surely had the intention to kill his companion."