Musnad Ahmad 32:811

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from 'Ata' ibn Abi Maymunah, that he heard Anas ibn Malik saying: "The Messenger of Allah (peace and blessings of Allah be upon him) used to enter the privy, and I and a boy like me would carry a vessel of water and a spear-headed staff, and he would cleanse himself with the water."