Abu Dawud
Purification (Kitab Al-Taharah)
Book Number: 001 / Hadith Number: 45
Narated By Abu Hurayrah : When the Prophet (pbuh) went to the privy, I took to him some water in a small vessel or a skin, and he cleansed himself. He then wiped his hand on the ground. I then took to him another vessel for ablution.
Abu Dawud said: The tradition transmitted by al-Aswad b. 'Amir is more perfect.