Musannaf Ibn Abi Shayba 18:690

Abu Bakr narrated to us, he said: Ghundar narrated to us, from Shu'bah, who said: I asked al-Hakam about the intentional [killing/injury] for which retaliation cannot be exacted. He said: "It is upon the 'aqilah (the paternal relatives)." And I asked Hammad, and he said: "It is from his [the perpetrator's] wealth."