Sunan Ibn Majah Book 1, Hadith 147

"I saw `Uthman and `Ali performing ablution, washing each part three times, and they said: 'This is how the Messenger of Allah used to perform ablution.'" (Hasan)

Another chain with similar wording.

Previous Hadith

"I saw the Messenger of Allah during the campaign of Tabuk performing ablution, washing each part once."

Sunan Ibn Majah Book 1, Hadith 146
Next Hadith

Performed ablution washing each part three times, and he attributed that to the Prophet.

Sunan Ibn Majah Book 1, Hadith 148