If a man – or he said someone – peeped at you without your permission and you threw a pebble at him that gouged out his eye, you would not be at fault.
Authentic hadithعن أبي هُريرة -رضي اللهُ عنه- مرفوعًا: «لو أن رجلا -أو قال: امْرَأً- اطَّلَعَ عليك بغير إِذْنِكَ؛ فَحَذَفْتَهُ بحَصَاةٍ، فَفَقَأْتَ عينه: ما كان عليك جُنَاحٌ».
Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "If a man – or someone – peeped at you without your permission and you threw a pebble at him that gouged out his eye, you would not be at fault."
Explanation
The Prophet (may Allah's peace and blessings be upon him) says that if someone looks at another from behind a door, from above a wall or something, without his permission, and then one gouges out his eye with a pebble or a skewer, this physical damage will not incur a sin or retribution on its doer. This is because the peeper is the aggressor and guilty for violating the other person's privacy.
Source · Narrated by Bukhari & Muslim
From the Hadith Encyclopedia (hadeethenc.com).