Sahih Muslim Book 3, Hadith Number 672.

Chapter : Utmost care for keeping private parts of body concealed.

Al-Miswar b. Makhrama reported: I was carrying a heavy stone and my lower garment was loose, and it, therefore, slipped off (so soon) that I could not place the stone (on the ground) and carry to its proper place. Upon this the Messenger of Allah (may peace be upon him) said: Return to your cloth (lower garment), take it (and tie it around your waist) and do not walk naked.