Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: "The Messenger of Allah (peace and blessings be upon him) used to take his ritual bath (ghusl) behind screens, and no one ever saw his 'awrah (private parts) (peace and blessings be upon him)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11080