Musnad Ahmad 626:8

'Abdah ibn Sulayman Abu Muhammad al-Kilabi narrated to us, Mujalid narrated to us, from al-Sha'bi, from al-Mughirah ibn Shu'bah, who said: I performed wudu for the Prophet, may Allah bless him and grant him peace, during a journey. He washed his face and his forearms, wiped over his head, and wiped over his leather socks. I said: "O Messenger of Allah, shall I not remove your leather socks?" He said: "No, for I inserted them while they were both in a state of purity, and I have not walked barefoot since." Then he performed the morning prayer.