Abu Dawud Book 34, Hadith Number 4491.
Chapter : If the legal heir of a man who has been killed intentionally agrees to accept bloodwit the slayer will be asked to pay him.
Narated By Abdullah ibn Amr ibn al-‘As : The Prophet (PBUH) said: A believer will not be killed for an infidel. If anyone kills a man deliberately, he is to be handed over to the relatives of the one who has been killed. If they wish, they may kill, but if they wish, they may accept blood-wit.