Narrated Jabir bin `Abdullah (ra):
While Allah’s Messenger (saws) (during his childhood) was carrying stones (along) with the people of Mecca for (the building of) the Ka`ba wearing an Izar (waist-sheet cover), his (paternal) 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 (Prophet pbuh) took off his Izar and put it over his shoulders, but he (Prophet pbuh) fell unconscious and since then he had never been seen naked.
Sahih al-Bukhari 364
In-book : Book 8, Hadith 16