Musannaf Ibn Abi Shayba 18:812

Abu Bakr narrated to us, he said: Abd al-Rahman ibn Mahdi narrated to us, from Hammad ibn Salamah, from Yahya ibn Sa'id, that Umar ibn Abd al-Aziz made the crime of the insane person [the responsibility of] the *'aqilah* (the paternal relatives who pay blood money).