Musnad Ahmad 1153:779

Aswad and Hajjaj narrated to us—the meaning is the same—they said: Shareek narrated to us, from al-Miqdam ibn Shurayh, from his father, who said: I asked Aisha about wiping over the leather socks (khuffayn). She said: "Go to Ali and ask him." He said: So I went to him, and he said: "The Prophet, may Allah bless him and grant him peace, used to command us, when we traveled, to wipe over our leather socks." Aswad said in his narration: And perhaps Shareek said: "When we were with the Prophet, may Allah bless him and grant him peace, on a journey, we would wipe over our leather socks."