It is narrated from Humran (may Allah have mercy on him) that Sayyiduna Uthman (may Allah be pleased with him) performed ablution three times at the place of sitting (al-Maqa'id) and said: This is how I saw the Messenger of Allah (peace and blessings be upon him) perform ablution, and I heard the Messenger of Allah (peace and blessings be upon him) say: "Whoever performs ablution well and then prays, Allah Almighty forgives him (the sins) between it and the next prayer, until he performs that prayer."
(b) Imam al-Shafi'i (may Allah have mercy on him) took this hadith as evidence for repeating the wiping. This narration is unqualified, while the explanatory narrations are those whose narrator is Humran (may Allah have mercy on him); in them repetition is mentioned for the limbs other than the head.
(c) Imam Abu Dawud al-Sijistani (may Allah have mercy on him) said: All the authentic narrations of Sayyiduna Uthman (may Allah be pleased with him) indicate that the wiping of the head is once, for they mentioned the ablution as three times each, and said in them: 'and he wiped his head', without mentioning a number as they did for the other limbs.
(d) The Shaykh (may Allah have mercy on him) said: In narrations reported from Sayyiduna Uthman (may Allah be pleased with him) through gharib (unusual) chains, repetition of the wiping is mentioned; but these narrations contradict the trustworthy hafiz narrators and are not a proof according to the hadith scholars, even though some of our colleagues have taken them as evidence.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 292