Sunan Abi Da'ud

Narrated Abu Hurayrah : When the Prophet (sal Allaahu alayhi wa sallam ) went to the privy, I took to him 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 and he performed ablution.Abu Dawud said : The tradition is transmitted by al-Aswad b. 'Amir is more perfect.