Musnad Ahmad 626:63

Ishaq ibn Yusuf narrated to us, Zakariya ibn Abi Za'idah narrated to us, from al-Sha'bi, from 'Urwah ibn al-Mughirah, from his father, who said: I was with the Prophet, peace and blessings of Allah be upon him, one night on a journey. He said, "Do you have water with you?" I said, "Yes." He descended from his riding beast, then walked until he disappeared from me in the darkness of the night. Then he came, and I poured [water] upon him from the water skin. He washed his face, and he was wearing a woolen cloak with tight sleeves, so he could not extract his forearms from them, so he brought them out from beneath the cloak. He washed his forearms and wiped over his head. Then I reached down to remove his leather socks, but he said, "Leave them, for I inserted them while they were in a state of purity." So he wiped over them.