Musnad Abu Ya'la al-Mawsili 125:5

Abu Khaythama narrated to us, Isma'il narrated to us, Rawh narrated to me, from 'Ata' ibn Abi Maymuna, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) would go out to relieve himself, and I would bring him water, and he would wash with it.