Musannaf Ibn Abi Shayba 18:658

Abu Bakr narrated to us, he said: Ibn 'Uyaynah narrated to us, from al-Aswad ibn Qays, from the tribe, that a young boy from his people entered upon a she-camel of Zayd ibn Sawhan in his house, and it struck him and killed him. His father came with a sword and hamstrung it. That was raised to Umar, so he declared the blood of the boy to be of no consequence (i.e., no retaliation) and held his father liable for the price of the she-camel.