Musannaf Ibn Abi Shayba 18:1165

Abu Bakr narrated to us, he said: Abu Usama narrated to us, from Awf, who said: Umar ibn Hayyan al-Hummani used to train horses. He mounted his son on a horse, and he fell from the horse and died. His blood money (diyah) was placed upon his paternal relatives (aqilah) during the time of Ziyad in Basra.