Musnad Ahmad 1153:1798

Isma'il narrated to us, Yunus narrated to us, from al-Hasan, who said: A man said: I said to Aisha: "What used to suffice the Messenger of Allah, may Allah bless him and grant him peace, for his ghusl (ritual bath) from janabah (major ritual impurity)?" She called for a vessel, which she estimated to be a sa' (a unit of measurement) equivalent to this sa' of yours.