Musannaf Ibn Abi Shayba 18:1347

Abu Bakr narrated to us, he said: Abdullah ibn Idris narrated to us, from Husayn, from Amir, who said: A dispute was brought to Ali regarding a bull that gored a donkey and killed it. Ali said: "If the bull entered upon the donkey and killed it, then he (the owner of the bull) is liable. And if the donkey entered upon the bull and it killed it, then there is no liability upon him."