Musnad Ahmad 1153:2374

Hassan ibn Musa narrated to us, Shaiban narrated to us, from Qatadah, from al-Hasan, from his mother, from Aisha, the wife of the Prophet, may Allah bless him and grant him peace, who said: "The Messenger of Allah, may Allah bless him and grant him peace, used to perform ghusl (full ablution) with a sa' of water, and perform wudu (ablution) with a mudd."