Sunan Abi Dawud 4017

Bahz b. Hakim said that his father told on the authority of his grandfather: I said: Messenger of Allah, from whom should we conceal our private parts and to whom can we show? He replied: conceal your private parts except from your wife and from whom your right hands possess (slave-girls). I then asked: Messenger of Allah, (what should we do), if the people are assembled together? He replied: If it is within your power that no one looks at it, then no one should look at it. I then asked: Messenger of Allah if one of us is alone, (what should he do)? He replied: Allah is more entitled than people that bashfulness should be shown to him.