Musnad Abu Ya'la al-Mawsili 252:1

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