Musnad Ahmad 70:1

Yunus ibn Muhammad narrated to us, 'Amr ibn Kathir al-Makki informed us, he said: I asked 'Abd al-Rahman ibn Kaysan, the freed slave of Khalid ibn Usayd, I said: "Will you not tell me about your father?" He said: "What did you ask me?" He said: My father told me that he saw the Messenger of Allah (peace and blessings of Allah be upon him) come out from the kitchens until he reached the well, while he was wearing an *izar* (waist wrapper) and had no *rida* (upper garment) on him. He saw slaves praying by the well, so he loosened the *izar* and wrapped it around his shoulders (as a *wisha'*), and he prayed two rak'ahs. I do not know if it was Dhuhr or 'Asr.