It is narrated from Sayyiduna Sahl bin Sa'd (may Allah be pleased with him) that a man peeped through a hole in the door of the Messenger of Allah (peace and blessings be upon him), while the Messenger of Allah (peace and blessings be upon him) was scratching his head with a comb. So the Messenger of Allah (peace and blessings be upon him) said: "Had I known that you were waiting for 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 / 5644