Sayyiduna Sahl bin Sa'd (may Allah be pleased with him) narrates that a man peeped through a hole in the door of the Prophet (peace and blessings be upon him), while the Messenger of Allah (peace and blessings be upon him) had a comb with which he (peace and blessings be upon him) was scratching his head. When the Messenger of Allah (peace and blessings be upon him) saw him, he said: "Had I known that you were seeing me, I would have thrust this into your eye. Seeking permission has only been prescribed because of the sight."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5650