(22) بَابُ: {وَإِنْ طَائِفَتَانِ مِنَ الْمُؤْمِنِينَ اقْتَتَلُوا فَأَصْلِحُوا بَيْنَهُمَا} فَسَمَّاهُمُ الْمُؤْمِنِينَ
Narrated Al-Ahnaf bin Qais:
While I was going to help this man ('Ali Ibn Abi Talib), Abu Bakra metme 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.' Isaid, '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."
Reference: Sahih al-Bukhari 31
In-book reference: Book 2, Hadith 24
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 1, Book 2, Hadith 31