Musnad Ahmad 32:375

Muhammad ibn Ja'far narrated to us, he said: Shu'bah narrated to us, from 'Abdullah ibn 'Abdullah ibn Jabr, that he heard Anas ibn Malik narrating: "That the Messenger of Allah, peace and blessings of Allah be upon him, used to perform ghusl—he and a woman from among his wives—from a single vessel."