Musnad Ahmad 1013:7

Sufyan narrated to us, from al-Zuhri, from Sahl ibn Sa'd: A man peeked through a hole in the chamber of the Prophet, peace and blessings of Allah be upon him, while he had a comb with which he was scratching his head. He said: "If I had known you were looking, I would have poked your eye with it. Indeed, seeking permission was only instituted because of the sight."