Abu Dawud Book 1, Hadith Number 94.

Chapter : The quantity of water that is desirable for ablution.

Narated By Umm Umarah : Habib al-Ansari reported: I heard Abbad ibn Tamim who reported on the authority of my grandmother, Umm Umarah, saying: The Prophet (PBUH) wanted to perform ablution. A vessel containing 2/3 mudd of water was brought to him.