Sahih Bukhari Volume 009, Book 083, Hadith Number 014.
Narrated By 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 Apostle 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 Apostle! It is alright for the killer, but what about the killed one?' He said, 'The killed one was eager to kill his opponent."