Musnad Abu Ya'la al-Mawsili 80:2

Ibrahim ibn al-Hajjaj narrated to us, Abd al-Aziz ibn al-Mukhtar narrated to us, Asim al-Ahwal narrated to us, Abdullah ibn Sarjis narrated that the Prophet, peace and blessings of Allah be upon him, forbade that a man should perform ghusl (ritual bath) with the leftover water of a woman, and that a woman should perform ghusl with the leftover water of a man. Rather, they should both dip into it together.