Musnad Ahmad 1153:417

'Abd al-Samad narrated to us, he said: Shu'bah narrated to us, he said: Abu Bakr ibn Hafs narrated to us, he said: I heard Abu Salamah say: I and the foster brother of 'Aishah entered upon 'Aishah. Her brother asked her about the ghusl (ritual washing) of the Messenger of Allah, may Allah bless him and grant him peace. She called for a vessel containing about a sa', and she performed ghusl and poured [water] over her head three times, and there was a veil between us and her.