Musannaf Ibn Abi Shayba 18:1189

Abu Bakr narrated to us, he said: Hafs narrated to us, he said: I asked Amr, "What did al-Hasan say regarding the *mu'ahid* (covenanted person) who is killed?" He said: "If they pay blood money among themselves, then it is upon the *'aqilah* (the paternal relatives). And if they do not pay blood money among themselves, then it is a debt upon his wealth and his liability."