Musannaf Ibn Abi Shayba 18:940

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Sharik narrated to us, from Mughira, from Ibrahim and al-Sha’bi, they both said: "Upon him is the blood money (diyah), and it is reduced from him according to the extent of the head wound (shajjah)."