Musnad Ahmad 792:19

Yahya ibn Sa'id and Isma'il ibn Ibrahim narrated to us, from Bahz, who said: My father narrated to me, from my grandfather, who said: I said: "O Messenger of Allah, regarding our private parts, what of them should we cover and what should we leave?" He said: "Guard your private part except from your wife or what your right hand possesses." I said: "O Messenger of Allah, what if the people are among one another?" He said: "If you are able that no one sees it, then let no one see it." I said: "What if one of us is alone?" He said: "Allah is more deserving that one should be shy before Him."