Musnad Ahmad 1153:997

An-Nadr ibn Isma'il Abu al-Mughirah narrated to us, he said: Ibn Abi Layla narrated to us, from 'Ata', he said: 'Aisha said: "The Messenger of Allah, peace and blessings of Allah be upon him, used to perform ablution (wudu) with a mudd, and perform the ritual bath (ghusl) with a sa'."