Musnad Ahmad 32:1772

'Affan narrated to us, Shu'bah informed us, from 'Abdullah ibn 'Abdullah ibn Jabr, who said: I heard Anas saying: "The Messenger of Allah, peace and blessings of Allah be upon him, used to perform ablution (wudu) with a *makkuk* (a specific measure of volume). And he used to perform the ritual bath (ghusl) with five *makkuks*."