Musnad Ahmad 30:195

Sufyan narrated to us, from Abu al-Zinad, from al-A'raj, from Abu Hurairah, from the Prophet (peace and blessings of Allah be upon him), who said: "If a man were to look in—and he said once: If a person were to look in—without your permission, and you threw a pebble at him and put out his eye, there would be no blame upon you."