Abu Dawud
Purification (Kitab Al-Taharah)
Book Number: 001 / Hadith Number: 110
Narated By Shaqiq b. Salamah : I saw 'Uthman b. 'Affan (perform ablution). He washed his forearms three times and washed his head thrice. He the said: I saw the Messenger of Allah (pbuh) doing like that.
Abu Dawud said: Another version says: "He performed ablution three times only."