Abu Dawud Book 34, Hadith Number 4481.
Chapter : If authorities ask relatives of a slain to forgo their right what should they do.
Narated By Abu Shurayh al-Khuza’i : The Prophet (PBUH) said: If a relative of anyone is killed, or if he suffers khabl, which means a wound, he may choose one of the three things: he may retaliate, or forgive, or receive compensation. But if he wishes a fourth (i.e. something more), hold his hands. After this whoever exceeds the limits shall be in grave penalty.