Bahz bin Hakim narrates from his father, from his grandfather (Sayyiduna Mu'awiyah bin Haydah (may Allah be pleased with him)), who said: I said: "O Messenger of Allah, what is the ruling concerning covering our private parts - what of them must we cover and what may we leave?" He (peace and blessings be upon him) said: "Guard your private parts except from your wife or from what your right hand possesses (a slave-woman)." I said: "Tell me, what if people are (crowded) together with one another?" He (peace and blessings be upon him) said: "If you are able to ensure that no one sees them, then let no one see them at all." I said: "Tell me, what if one of us is alone?" He (peace and blessings be upon him) said: "Allah Almighty is more deserving that one should be modest before Him." And he (peace and blessings be upon him) placed his hand over his private parts. This hadith is sahih al-isnad (its chain is authentic), but the two Shaykhs (Bukhari and Muslim) did not record it.