Musannaf Ibn Abi Shayba 18:1078

Abu Bakr narrated to us, he said: Ibn Mahdi narrated to us, from Sufyan, from Umar bin Abd al-Rahman al-Sahmi, from a man, that a man came to Umar bin al-Khattab regarding a *mudihah* (a head wound that exposes the bone). He said: "We do not pay blood money (diya) among ourselves."