The Prophet (peace and blessings be upon him) said:
"Seeking permission (before entering someone's house) has in fact been prescribed because of the eyes (to protect them from seeing what is not permitted)."
Hadith Referenceصحیح الجامع الصغیر / حرف الألف / 2354
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[حم ق ت] عن سهل بن سعد. مختصر مسلم ١٤٢٤.