Musannaf Ibn Abi Shayba 18:919

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Sufyan narrated to us, from Mansur, from Ibrahim: That two young boys were playing, and one of them threw the other down, so one of them sustained a head wound and the other’s front tooth was broken. He (Ibrahim) held the one who was underneath liable for the one who was on top, and he did not hold the one who was on top liable for the one who was underneath.