Allaah says (interpretation of the meaning):
"O you who believe! Enter not houses other than your own, until you have asked permission and greeted those in them . . ."
The Prophet (peace and blessings of Allaah be upon him) explained that the reason why permission was to be sought was for fear that the person seeking to enter might see something private in the house:
"The rule of seeking permission has been established for the sake of (not) seeing."
(Reported by al-Bukhaari, see Fath al-Baari, 11/24).
Nowadays, when houses and buildings are too close to one another or even attached, and doors and windows face one another, the possibility of neighbours seeing one another has increased greatly.
Many people do not lower their gaze, and some of those who live on higher floors may deliberately look down from their roofs and windows into neighbouring homes that are lower than their own. This is an act of betrayal and an invasion of their neighbours privacy, as well as being the way that leads to haraam deeds.
A great deal of misery and trouble has resulted from this, and that fact that the Shareeah counts the eye of the one who spies as worthless is sufficient proof of the seriousness of the matter.
The Messenger of Allaah (peace and blessings of Allaah be upon him) said: "Whoever looks into somebodys house without their permission, it is permissible for them to put out his eye."
(Reported by Muslim, 3/1699). According to another report, he said: "Put out his eye, and there will be no penalty or retribution.
(Reported by Imaam Ahmad, 2/385; see also Saheeh al-Jaami', 6022).
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