Musannaf Ibn Abi Shayba 18:922

Abu Bakr narrated to us, he said: Abd al-Rahim narrated to us, from Hajjaj, from al-Hakam, from Ibrahim, from Ali ibn Abi Talib, who said: Two boys were playing, and one of them jumped onto the back of his companion. The upper one’s front tooth broke, and the lower one was wounded. So they held each other liable.