It is narrated from Sayyiduna Abu al-Tufayl (may Allah be pleased with him) that when the House (of Allah) was being built, the people were carrying stones, and the Prophet (peace and blessings be upon him) was also carrying stones with them. He (peace and blessings be upon him) took his cloth and placed it on his shoulder (to protect himself from the rubbing of the stones), and a voice called out (from the unseen): "Do not uncover your private parts." So he (peace and blessings be upon him) threw down the stone and put on his cloth. This hadith is sahih al-isnad (its chain is authentic), but the two Shaykhs (Bukhari and Muslim) did not record it.