Sahih Muslim Book 2, Hadith Number 529.

Chapter : Wiping over the socks.

‘Urwa b. Mughira reported his father having said: I was one night with the Apostle of Allah (may peace be upon him) on a journey. He said to me: Have you any water with you? I said: Yes. He (the Holy Prophet) came down from his ride and went on till he disappeared in the darkness of night. He then came back and I poured water for him from the jar. He washed his face, He had a woollen gown on him and he could not bring out his forearms from it (i.e. from its sleeves) and consequently he brought them out from under his gown. He washed his forearms, wiped over his head. I then bent down to take off his socks. But he said: Leave them, for my feet were clean when I put them in, and he only wiped over them.