Musnad Ahmad 1153:1089

Yazid narrated to us, he said: Shu'bah ibn al-Hajjaj informed us, from Abu Bakr ibn Hafs, from Abu Salamah, from 'Aishah, he said: Her foster brother asked her about the Messenger of Allah's (peace and blessings of Allah be upon him) bathing from major ritual impurity (janabah). So she called for water, the amount of a sa', and she bathed and poured it over her head three times.