Musnad Ahmad 1153:2269

'Abd al-Wahhab narrated to us, from Sa'id, from 'Asim al-Ahwal, from Mu'adhah, from 'A'ishah, who said: "I used to perform ghusl, I and the Messenger of Allah, from a single vessel." And it was in his hadith: "That the Prophet, peace and blessings of Allah be upon him, would begin before her."