Volume 9, Book 88, Number 204:
Narrated 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 in –
tend 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)