Musannaf Ibn Abi Shayba 18:1217

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Rabi’ah ibn ‘Uthman al-Taymi narrated to us, from Sa’id ibn Ibrahim, that Abu Musa wrote to ‘Umar: "A man dies before us and he has no relative (rahim) and no master (mawla)." He said: So ‘Umar wrote to him: "If he left a relative, then the relative [inherits], and if not, then the one to whom he had a pledge of mutual aid (al-wafa’), and if not, then the public treasury (bayt al-mal); they inherit from him and they pay blood money on his behalf."