Sahih Muslim 2158
Sahih Muslim · Book 38: The Book of Manners and Etiquette · The Prohibition Of Looking Into A House
Abu Huraira reported Allah's Messenger (ﷺ) as saying:
حَدَّثَنَا ابْنُ أَبِي عُمَرَ ، حَدَّثَنَا سُفْيَانُ ، عَنْ أَبِي الزِّنَادِ ، عَنِ الأَعْرَجِ ، عَنْ أَبِي هُرَيْرَةَ ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " لَوْ أَنَّ رَجُلاً اطَّلَعَ عَلَيْكَ بِغَيْرِ إِذْنٍ فَخَذَفْتَهُ بِحَصَاةٍ فَفَقَأْتَ عَيْنَهُ مَا كَانَ عَلَيْكَ مِنْ جُنَاحٍ " .
If a person were to cast a glance in your (house) without permission, and you had in your hand a staff and you would have thrust that in his eyes, there is no harm for you.