صحيح البخاري

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! It is alright for the killer, but what about the killed one?' He said, 'The killed one was eager to kill his opponent."