Musnad Ahmad 32:1167

Yazid ibn Harun narrated to us, Shu'bah informed us, from 'Ata' ibn Abi Maymunah, from Anas ibn Malik, who said: "When the Messenger of Allah (peace and blessings of Allah be upon him) would go out to relieve himself, I and a young boy would come to him with a water vessel and a short spear, and he would cleanse himself with water."