Sayyiduna Mu'awiyah bin Haydah (may Allah be pleased with him) narrated: I said: "O Messenger of Allah! What may we expose of our private parts and what must we cover?" He (peace and blessings be upon him) said: "Guard your private parts except from your wife and your slave woman." I said: "And when we are gathered together with one another?" He (peace and blessings be upon him) said: "Guard your private parts as much as you can." I said: "And when we are in seclusion?" He (peace and blessings be upon him) said: "Allah is more deserving that one should feel modesty before Him."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17041