Sahl bin Sa'd (May Allah be pleased with him) reported:
The Messenger of Allah (ﷺ) said, "Seeking permission to enter (somebody's house) has been prescribed in order to restrain the eyes (from looking at something we are not supposed to look at)."[Al-Bukhari and Muslim].
App reference: Book 6, Hadith 27
Arabic/English book reference: Book 6, Hadith 871