Abu Dawud Book 1, Hadith Number 45.
Chapter : Wiping ones hand on the ground after easing.
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.