Abu Dawud Book 30, Hadith Number 4260.

Chapter : Gravity of the sin of killing a beleiver.

Narated By Sa’id b. Jubair : I asked Ibn ‘Abbas (about the verse relating to intentional homicide in Surat al-Nisa’). He said: When the verse “Those who invoke not with Allah any other god, nor slay such life as Allah has made sacred, except for just cause (Qur’an xxv. 68)” was revealed, the polytheists of Mecca said: We have killed the soul prohibited by Allah, invoked another god along with Allah for worship, and commited shameful deed. So Allah revealed the verse “unless he repents, beleives and works righteous deeds, for Allah will change the evil of such persons into good. (Qur’an xxv. 70)” This is meant for them. As regards the verse “If a man kills a beleiver intentionally, his recompense is Hell (Quran iv. 93)” he said: If a man knows the command of Islam and intentionally kills a beleiver, his repentance will not be accepted. I then mentioned it to Mujahid. He said: Except the one who was ashamed (of his sin).