Musnad Abu Ya'la al-Mawsili 125:4

Abu Bakr ibn Abi Shaybah narrated to us, Waki’ and Ghundar narrated to us, from Shu’bah, from ‘Ata’ ibn Abi Maymunah, who said: I heard Anas saying: I used to go out with the Prophet, may Allah bless him and grant him peace, while I was a young boy, and with me was a water container and a short spear. He would relieve himself, then perform ablution.