Sahih al Bukhari 31

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 (ﷺ) 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 (ﷺ)! It is all right for the murderer but what about the murdered one?' Allah's Messenger (ﷺ) replied, "He surely had the intention to kill his companion

Sahih al Bukhari 31
Book 2, Hadith 24