Musnad Ahmad 1099:3

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from 'Abd Allah ibn 'Uthman ibn Khuthaym, from Abu al-Tufayl, who said: When the House (the Ka'bah) was being built, the people were carrying stones, and the Prophet, peace and blessings of Allah be upon him, was carrying with them. He took his garment and placed it upon his shoulder. Then it was called out: "Do not uncover your private parts." So he dropped the stone and put on his garment, peace and blessings of Allah be upon him.