Musnad Ahmad 32:1773

'Affan narrated to us, Shu'bah narrated to us, from Abu Mu'adh 'Ata' ibn Abi Maymunah, who said: I heard Anas saying: "When the Messenger of Allah (peace and blessings of Allah be upon him) would go out for his need, I and a young boy from among us would come with a vessel of water."