Musnad Ahmad 1018:71

Surayj narrated to us, Abu 'Awanah informed us, from Dawud ibn 'Abdullah al-Awdi, from Humayd ibn 'Abd al-Rahman, who said: I met a man who accompanied the Prophet, may Allah bless him and grant him peace, just as Abu Hurayrah accompanied him for four years. He said: "The Messenger of Allah, may Allah bless him and grant him peace, forbade us that any one of us should comb his hair every day, or urinate in his place of bathing, or that a woman should bathe with the leftover water of a man, or that a man should bathe with the leftover water of a woman; rather, they should both scoop [the water] together."