Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.046.
Section : The place to be wiped on a sock.
Islamic law prescribes that the top of the sock is to be wiped.
Said al-Mughirah, “I saw the Messenger of Allah, upon whom be peace, wipe over the top of his socks.” (Related by Ahmad, Abu Dawud and at-Tirmizhi, who called it hassan.) ‘Ali observed, “If the religion was based on opinion, the bottom of the sock would take preference in being wiped to the top of the sock.” (Related by Abu Dawud and ad-Daraqutni with a hassan or sahih chain.) What is obligatory in the wiping is what is meant by the lexicographical meaning of the word “wipe.” There are no specifications authentically mentioned with respect to the wiping.