Musannaf Ibn Abi Shayba 1:709

Ibn ‘Ulayyah narrated to us, from Yunus, from al-Hasan, that a man narrated to them, saying: I entered upon ‘Aishah. I said: "O Mother of the Believers, what did the Messenger of Allah (may Allah bless him and grant him peace) use to perform his ghusl (ritual bath) with?" He said: So she called for a vessel, which I estimated to be a sa’ of this sa’ of yours.