My foster brother, who was also Ayesha's foster brother, and I went in to see Ayesha. Her brother asked her about the way the Messenger of Allah ﷺ used to perform the major ablution. She called for a vessel holding about a saa, then bathed and poured water over her head three times, with a screen between us and her.
حدثنا عبد الصمد قال حدثنا شعبة قال حدثنا أبو بكر بن حفص قال سمعت
دخلت أنا وأخو عائشة من الرضاعة على عائشة فسألها أخوها عن غسل رسول الله ﷺ فدعت بإناء نحوا من صاع فاغتسلت وأفرغت على رأسها ثلاثا وبيننا وبينها الحجاب
Its chain of transmission is sahih. Bukhari reported it, 1/364, no. 251, Fath, in the chapter on ghusl. Muslim reported it, 1/256, no. 320, in the chapter on menstruation. Nasai reported it, 1/127, no. 227, in the chapter on ghusl. And Bayhaqi reported it, 1/195, in the chapter on purification.