Yazid bin Abd al-Aziz narrates this hadith from al-A'mash: "I used to think that the inner (lower) part of the feet was more deserving of wiping, until I saw the Messenger of Allah (peace and blessings be upon him); he (peace and blessings be upon him) would wipe only over the upper part of his khuffs (leather socks)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1388