Musnad Ahmad 626:102

Yahya ibn Sa'id narrated to us, from Zakariyya, from 'Amir, who said: 'Urwah ibn al-Mughirah narrated to me, 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 to me, "Do you have water with you?" I said, "Yes." He descended from his riding beast, then went away from me until he disappeared from me in the darkness of the night. He was wearing a cloak (jubbah). He went to take his arms out, but he could not take his arms out of it, so he took his arms out from under the cloak. He washed his arms and wiped his head. Then I went to remove his leather socks (khuffayn), and he said, "Leave them, for I inserted them while they were in a state of purity." So he wiped over them.