Sahih Bukhari | Volume 9 | Hadith Number 204

Narrated by Al-Hasan
(Al-Ahnaf said:) I went out carrying my arms during the nights of the affliction (i.e. the war between 'Ali and 'Aisha) and Abu Bakra met me and asked, "Where are you going?" I replied, "I intend to help the cousin of Allah's Apostle (i.e. 'Ali)." Abu Bakra said, "Allah's Apostle said, 'If two Muslims take out their swords to fight each other, then both of them will be from amongst the people of the Hell-Fire.' It was said to the Prophet, 'It is alright for the killer but what about the killed one?' He replied, 'The killed one had the intention to kill his opponent.'" (See Hadith No. 30, Vol. 1)