Musannaf Ibn Abi Shayba 18:865

Abu Bakr narrated to us, he said: Abd al-A’la narrated to us, from Ma’mar, from al-Zuhri, who said: Umar said regarding it: If a man forms a pact of brotherhood (muwalat) with a man, then he has his inheritance, and his 'aqilah (kinsmen) are responsible for his blood money.