Musnad Ahmad 1153:2001

Yazid narrated to us, he said: Hammam informed us, from Qatadah, from Safiyyah bint Shaybah, from Aisha, that the Messenger of Allah (peace and blessings of Allah be upon him) used to perform ablution (wudu) with the amount of a mudd, and perform ghusl with the amount of a sa’.