Musnad Ahmad 1013:37

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-Zuhri, from Sahl ibn Sa'd, that a man peeked at the Prophet, may Allah bless him and grant him peace, through a hole in the chamber. In the hand of the Prophet, may Allah bless him and grant him peace, was a comb. He said: "If I knew that you were watching me, I would have poked your eye with this comb. Indeed, permission was only made because of the sight."