Abu Dawud Book 1, Hadith Number 135.
Chapter : Washing the limbs in ablution three times.
Narated By Abdullah ibn Amr ibn al-‘As : A man came to the Prophet (PBUH) and asked him: Messenger of Allah, how is the ablution (to performed)?
He (the Prophet) then called for water in a vessel and washed his hands up to the wrists three times, then washed his face three times, and washed his forearms three times. He then wiped his head and inserted both his index fingers in his ear-holes; he wiped the back of his ears with his thumbs and the front of his ears with the index fingers. He then washed his feet three times.
Then he said: This is how ablution should be performed. If anyone does more or less than this, he has done wrong and transgressed, or (said) transgressed and done wrong.