Musannaf Ibn Abi Shayba 1:200

Abu Khalid al-Ahmar narrated to us, from Yahya ibn Sa'id, from Muhammad ibn Mahmud, who said: The Messenger of Allah (may Allah bless him and grant him peace) saw a blind man performing ablution, and he washed his face and his hands. The Prophet (may Allah bless him and grant him peace) began saying, "The soles of your feet." So he began washing the soles of his feet.