(a) It is reported from Sayyiduna Ali (may Allah be pleased with him) that he performed ablution: he washed his face and his hands three times, wiped his head three times and washed his feet three times. Then he said: "This is how I saw the Messenger of Allah (peace and blessings be upon him) perform ablution." Ibn Wahb also says the same: he wiped his head three times.
(b) In the narration of Ibn Jurayj it is: He wiped his head once.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 298