Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.031B.

This is a sunnah that the Prophet, upon whom be peace, almost always followed. If he acted otherwise, it was just to show that the other acts are permissible. ‘Amr ibn Shu’aib related on the authority of his father from his grandfather who said: “A bedouin came to the Messenger of Allah, upon whom be peace, and asked him about the ablution. He showed him how to wash each part three times and said, ‘This is the ablution. Whoever does more than that has done wrong, transgressed and committed evil.” (Related by Ahmad, an-Nasa’i and Ibn Majah.) ‘Uthman also reported that the Messenger of Allah would repeat each washing three times. (Related by Ahmad, Muslim and at-Tirmizhi.) It is also proven that he performed each washing only once or twice. According to most of the reports, he wiped his head only once.