Musannaf Ibn Abi Shayba 1:1242

Abu Usama narrated to us, from Shu'bah, from 'Amarah ibn Abi Hafsah, who said: I heard Abu Mijlaz saying: I said to Ibn 'Umar: I sent my camel, and it urinated, and its urine touched me. He said: "Wash it." I said: "It only splashed such and such," meaning I was downplaying it. He said: "Wash it."