Sahih Muslim Book 003, Hadith Number 0670.
Chapter : Utmost care for keeping private parts of body concealed.
Jabir b. 'Abdullah reported: When the Ka'ba was constructed the Apostle of Allah (may peace be upon him) and Abbas went and lifted stones. Abbas said to the Messenger of Allah (may peace be upon him): Place your lower garment on your shoulder (so that you may protect yourself from the roughness and hardness of stones). He (the Holy Prophet) did this, but fell down upon the ground in a state of unconsciousness and his eyes were turned towards the sky. He then stood up and said: My lower garment, my lower garment; and this wrapper was tied around him. In the Hadith transmitted by Ibn Rafi', there is the word: "On his neck" and he did not say: "Upon his shoulder."