The Prophet ﷺ said: When one has a look into the house, then there is no (need of) permission. (Using translation from Abū Dāʾūd 5173)   

أَنَّ النَّبِيَّ ﷺ قَالَ إِذَا دَخَلَ الْبَصَرُ فَلَا إِذْنَ  

[Machine] Sa'd ibn Mu'adh came to the Prophet ﷺ and sought permission to enter while he was facing the door. The Prophet ﷺ said with his hand like this, "O Sa'd, seeking permission is by just looking."  

أَتَى سَعْدُ بْنُ مُعَاذٍ النَّبِيَّ ﷺ فَاسْتَأْذَنَ عَلَيْهِ وَهُوَ مُسْتَقْبِلٌ الْبَابَ فَقَالَ النَّبِيُّ ﷺ بِيَدِهِ هَكَذَا يَا سَعْدُ فَإِنَّمَا الِاسْتِئْذَانُ مِنَ النَّظَرِ  

[Machine] Saad sought permission to meet the Prophet Muhammad ﷺ near the door, and the Prophet said to him, "When seeking permission, do not face the door, as both of them are sent."  

أَنَّ سَعْدًا اسْتَأْذَنَ عَلَى النَّبِيِّ ﷺ قُبَالَةَ الْبَابِ فَقَالَ لَهُ إِذَا اسْتَأْذَنْتَ فَلَا تَسْتَقْبِلِ الْبَابَ كِلَاهُمَا مُرْسَلٌ  

[Machine] The Messenger of Allah ﷺ, when he came to the door of a people, would walk along the wall and not face the door. Instead, he would stand to the right and left and seek permission. If permission was granted, he would enter, and if not, he would turn back. This is because the people did not have curtains for their doors. This is the wording of the Hadith of Adam, and in another narration by Al-Harrani, he did not face the door directly, but rather from his right or left corner. He would say, "Peace be upon you." This is because the doors at that time did not have doorknobs.  

كَانَ رَسُولُ اللهِ ﷺ إِذَا أَتَى بَابَ قَوْمٍ مَشَى مَعَ الْجِدَارِ وَلَمْ يَسْتَقْبِلِ الْبَابَ وَلَكِنْ يَقُومُ يَمِينًا وَشِمَالًا فَيَسْتَأْذِنُ فَإِنْ أُذِنَ لَهُ وَإِلَّا رَجَعَ وَذَلِكَ أَنَّ الْقَوْمَ لَمْ يَكُنْ لِأَبْوَابِهِمْ سُتُورٌ هَذَا لَفْظُ حَدِيثِ آدَمَ وَفِي رِوَايَةِ الْحَرَّانِيِّ لَمْ يَسْتَقْبِلِ الْبَابَ مِنْ تِلْقَاءِ وَجْهِهِ وَلَكِنْ مِنْ رُكْنِهِ الْأَيْمَنِ أَوِ الْأَيْسَرِ وَيَقُولُ السَّلَامُ عَلَيْكُمْ وَذَلِكَ أَنَّ الدُّورَ لَمْ يَكُنْ عَلَيْهَا يَوْمَئِذٍ سُتُورٌ