Musnad Abu Ya'la al-Mawsili 129:1

Abu Bakr ibn Abi Shaybah narrated to us, Hushaym narrated to us, Humayd al-Tawil informed us, from Anas ibn Malik, that the Prophet, peace and blessings of Allah be upon him, used to go around to all of his wives in one night with a single ritual bath (ghusl).