Musnad Ahmad 32:1411

Haywah ibn Shurayh narrated to us, Baqiyyah narrated to us, Shu'bah narrated to us, from Hisham ibn Zayd, from Anas, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to go around to his wives with a single ritual bath (ghusl)."