It is narrated from Sayyiduna Ali bin Abi Talib (may Allah be pleased with him): I used to think that the inner (that is, lower) part of the feet was more deserving of wiping than the outer part, until I saw the Messenger of Allah (peace and blessings be upon him); he (peace and blessings be upon him) performed ablution and wiped over the upper part of his khuffs (leather socks). (b) All these narrations that mention the khuffs are evidence that it (the other wording) was abridged.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1389