It is narrated from Sayyiduna Ali (may Allah be pleased with him): If the religion were based on opinion, wiping the bottom of the khuff (leather sock) would be more fitting than its top, yet I saw the Messenger of Allah (peace and blessings be upon him); he (peace and blessings be upon him) used to wipe like this with his fingers (that is, he demonstrated it practically).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1387