Sahih Bukhari, Chapter: Prayers (Salat)

While Allah's Apostle was carrying stones (along) with the people of Mecca for (the building of) the Ka'ba wearing an Izar (waist-sheet cover), his uncle Al-'Abbas said to him, "O my nephew! (It would be better) if you take off your Izar and put it over your shoulders underneath the stones." So he took off his Izar and put it over his shoulders, but he fell unconscious and since then he had never been seen naked.