Abu Dawud Book 30, Hadith Number 4259.
Chapter : Gravity of the sin of killing a beleiver.
Narated By Zayd ibn Thabit : The verse “If a man kills a believer intentionally, his recompense is Hell to abide therein for ever” was revealed six months after the verse “And those who invoke not with Allah any other god, nor slay such life as Allah has made sacred, except for just cause in Surat al-Furqan.