Musnad Abu Ya'la al-Mawsili 108:178

Dawud ibn Rashid narrated to us, Baqiyyah narrated to us, from Jarir ibn Yazid al-Himyari, al-Mundhir narrated to me, from Muhammad ibn al-Munkadir, from Jabir ibn Abdullah, who said: The Messenger of Allah (may Allah bless him and grant him peace) passed by a man who was performing ablution and was washing his leather socks (khuffayn). He poked him with his hand and said, "We were not commanded to do this." He said: Then the Messenger of Allah (may Allah bless him and grant him peace) showed him, and he wiped with his hand from the front of the leather socks to the shin, and he spread his fingers, one time.