Musnad Ahmad 937:281

Yahya ibn Ishaq narrated to us, Ibn Lahi'ah informed us, and Musa narrated to us, Ibn Lahi'ah narrated to us, from Ubayd ibn Abi Ja'far, from Abu Abd al-Rahman al-Hubuli, from Abu Dharr, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Any man who lifts a curtain and inserts his gaze before he is given permission has committed a transgression that is not lawful for him to commit. If a man were to gouge out his eye, it would be unpunished (blood-money would not be due). And if a man passes by a door that has no curtain and sees the private parts of its inhabitants, there is no sin upon him; the sin is only upon the people of the house."