Sunan Ibn Majah
270
Sahih+2

Abdullah bin Muhammed bin 'Aqil bin Abu Talib narrated from his father that his grandfather said:"The Messenger of Allah said: 'A Mudd is sufficient for the ablution and a Sa' is sufficient for the bath.' A man said: 'It is not sufficient for us.'" He (the narrator) said: "It was sufficient for one who is better than you and had more hair" meaning the Prophet

SahihAl-Albani
SahihMuhammad Fouad Abd al-Baqi