Sahih al-Bukhari 31
Sahih al-Bukhari · Book 2: Belief · "And if two parties (or groups) from among the believers fall to fighting, then make peace between them both…" Allah has called them "believers"
Narrated Al-Ahnaf bin Qais:
حَدَّثَنَا عَبْدُ الرَّحْمَنِ بْنُ الْمُبَارَكِ ، حَدَّثَنَا حَمَّادُ بْنُ زَيْدٍ ، حَدَّثَنَا أَيُّوبُ ، وَيُونُسُ ، عَنِ الْحَسَنِ ، عَنِ الأَحْنَفِ بْنِ قَيْسٍ ، قَالَ ذَهَبْتُ لأَنْصُرَ هَذَا الرَّجُلَ، فَلَقِيَنِي أَبُو بَكْرَةَ فَقَالَ أَيْنَ تُرِيدُ قُلْتُ أَنْصُرُ هَذَا الرَّجُلَ. قَالَ ارْجِعْ فَإِنِّي سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ " إِذَا الْتَقَى الْمُسْلِمَانِ بِسَيْفَيْهِمَا فَالْقَاتِلُ وَالْمَقْتُولُ فِي النَّارِ ". فَقُلْتُ يَا رَسُولَ اللَّهِ هَذَا الْقَاتِلُ فَمَا بَالُ الْمَقْتُولِ قَالَ " إِنَّهُ كَانَ حَرِيصًا عَلَى قَتْلِ صَاحِبِهِ " .
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."