Musnad Ahmad 32:160

Isma'il ibn Ibrahim narrated to us, Rawh ibn al-Qasim narrated to 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 would bring him water, and he would wash with it."