Musnad Ahmad 1153:702

Abu Sa'id narrated to us, he said: Abu 'Awanah narrated to us, he said: 'Umar narrated to us, from his father, who said: I said to 'Aishah: "Did you used to perform ghusl (ritual bath) with the Prophet, may Allah bless him and grant him peace?" She said: "Yes, the Messenger of Allah, may Allah bless him and grant him peace, and I used to perform ghusl from a single vessel."