Musnad Ahmad 5:385

Hajjaj narrated to us, Shareek narrated to us, from al-Miqdam ibn Shurayh, from his father, who said: I asked Aisha, may Allah be pleased with her, and I said: "Inform me of a man from the companions of the Prophet, may Allah bless him and grant him peace, whom I may ask about wiping over the leather socks." She said: "Go to Ali and ask him, for he used to accompany the Prophet, may Allah bless him and grant him peace." He said: So I went to Ali and asked him, and he said: "The Messenger of Allah, may Allah bless him and grant him peace, commanded us to wipe over our leather socks when we traveled."