Sahih al-Bukhari 6875
Narrated Al-Ahnaf bin Qais:
I went to help that man (i.e., `Ali), and on the way I met Abu Bakra who asked me, "Where are you 
going?" I replied, "I am going to help that man." He said, "Go back, for I heard Allah's Messenger (ﷺ) 
saying, 'If two Muslims meet each other with their swords then (both) the killer and the killed one are 
in the (Hell) Fire.' I said, 'O Allah's Messenger (ﷺ)! It is alright for the killer, but what about the killed one?' 
He said, 'The killed one was eager to kill his opponent."
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 ‏"‏‏.‏ قُلْتُ يَا رَسُولَ اللَّهِ هَذَا الْقَاتِلُ فَمَا بَالُ الْمَقْتُولِ قَالَ ‏"‏ إِنَّهُ كَانَ حَرِيصًا عَلَى قَتْلِ صَاحِبِهِ ‏"‏‏.‏