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].

وعن سهل بن سعد قال: قال رسول الله ﷺ : "إنما جعل الاستئذان من أجل البصر" ((متفق عليه)).