In the name of Allah, the Most Gracious, the Most Merciful.
Quran: Chapter 18 ‘Qad Aflaha’ ‘Successful indeed are’, Sura: 24 ‘An Nur’ ‘The Light’, Verse: 27- 28
Verse 27: O you who believe! Enter not houses other than your own, until you have asked permission and greeted those in them; that is better for you, in order that you may remember.
Verse 28: And if you find no one therein, still, enter not until permission has been given. And if you are asked to go back, go back, for it is purer for you. And Allah is All-Knower of what you do.
Notes by samia2010oct: Before entering houses of friends, relatives, and colleagues etc. etc., permission should be taken from the people of the house and when entering greet the inhabitants by saying, ‘Assalaam alaikum’, ‘peace be on you’ loud enough so that people inside hear it clearly. If people of the house are not in their home one must not enter at all. If people are inside but permission to enter is not granted, one must come back peacefully without feeling bad about it and one must never think badly about the inhabitants of the house.
Trespassing is Big Sin, Gunaah e Kabira. In case permission is not granted and visitor misbehaves or thinks ill of the inhabitants these are Big Sins as well. For Big Sins there is the punishment of unbearable burning of whole body by Fire.