Abu Dawud Book 26, Hadith Number 4006.

Narated By Mu’awiyah ibn Haydah : I said: Apostle 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: Apostle 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: Apostle 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.