Abu Dawud Book 30, Hadith Number 4255.
Chapter : Prohibition of fighting during the period of commotion.
Narated By Ahnaf b. Qais : I came out with the intention of (participating in) fighting. Abu Bakrah met me and said: Go back, for I heard the Apostle of Allah (PBUH) say: When two muslims face each other with their swords, the killer and the slain will go to Hell. He asked: Apostle of Allah, this is the killer (so naturally he should go to the Hell), but what is the matter with the slain? He replied: He intended to kill his companion.