Musannaf Ibn Abi Shayba 35:208

Khalid ibn Makhlad narrated to us, from Sulayman ibn Bilal, from Suhayl, from his father, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If a man looks in upon a people without their permission, it is permissible for them to gouge out his eye."