Allah the Exalted said:
رياض الصالحينAbū Zakariyyā Muḥyī al-Dīn Yaḥyā ibn Sharaf al-Nawawī١٤٠ - باب الاستئذان وآدابه
١٤٠ - باب الاستئذان وآدابه
O you who have believed, do not enter houses other than your own houses until you ascertain welcome and greet1 their inhabitants. That is best for you; perhaps you will be reminded [i.e., advised].
And the Exalted also said:
وَإِذَا بَلَغَ ٱلْأَطْفَـٰلُ مِنكُمُ ٱلْحُلُمَ فَلْيَسْتَـْٔذِنُوا۟ كَمَا ٱسْتَـْٔذَنَ ٱلَّذِينَ مِن قَبْلِهِمْ ۚ كَذَٰلِكَ يُبَيِّنُ ٱللَّهُ لَكُمْ ءَايَـٰتِهِۦ ۗ وَٱللَّهُ عَلِيمٌ حَكِيمٌ
And when the children among you reach puberty, let them ask permission [at all times] as those before them have done. Thus does Allāh make clear to you His verses; and Allāh is Knowing and Wise.
عن أَبي موسى الأشعري - ﵁ - قَالَ: قَالَ رسول الله - ﷺ: «الاسْتِئْذَانُ ثَلاثٌ، فَإنْ أُذِنَ لَكَ وَإِلاَّ فَارْجِعْ». متفقٌ عَلَيْهِ.
Abū Mūsā al-Ashʿarī (may Allah be pleased with him) reported: The Messenger of Allah ﷺ said: "Asking permission to enter is three times; if you are granted permission, then [enter], and if not, then go back." Agreed upon (al-Bukhārī and Muslim).
وعن سهلِ بنِ سعدٍ - ﵁ - قَالَ: قَالَ رسول الله - ﷺ: «إنَّمَا جُعِلَ الاسْتِئذَانُ مِنْ أَجْلِ البَصَرِ». متفقٌ عَلَيْهِ.
Sahl ibn Saʿd (may Allah be pleased with him) reported: The Messenger of Allah ﷺ said: "Asking permission to enter was ordained only on account of the gaze." Agreed upon (al-Bukhārī and Muslim).
وعن رِبْعِيِّ بن حِرَاشٍ، قَالَ: حَدَّثَنَا رَجُلٌ مِنْ بَنِي عَامِرٍ أنَّهُ اسْتَأذَنَ عَلَى النَّبيِّ - ﷺ - وَهُوَ في بيتٍ، فَقَالَ: أألِج؟ فَقَالَ رسول الله - ﷺ - لِخَادِمِهِ: «أُخْرُجْ إِلَى هَذَا فَعَلِّمهُ الاسْتِئذَانَ، فَقُلْ لَهُ: قُلِ: السَّلاَمُ عَلَيْكُمْ، أأدْخُل؟» فَسَمِعَهُ الرَّجُلُ، فَقَالَ: السَّلامُ عَلَيْكُمْ، أَأَدْخُل؟ فَأذِنَ لَهُ النَّبيُّ - ﷺ - فدخلَ. رواه أَبُو داود بإسناد صحيح.
Ribʿī ibn Ḥirāsh reported: A man from Banū ʿĀmir narrated to us that he sought permission to enter upon the Prophet ﷺ while he was in a house, and he said: "May I come in?" So the Messenger of Allah ﷺ said to his servant: "Go out to this man and teach him how to ask permission; say to him: 'Say: Peace be upon you—may I enter?'" The man heard him and said: "Peace be upon you—may I enter?" So the Prophet ﷺ granted him permission, and he entered. Narrated by Abū Dāwūd with a sound chain (isnād).
عن كِلْدَةَ بن الحَنْبل - ﵁ - قَالَ: أتَيْتُ النبيَّ - ﷺ - فَدَخَلْتُ عَلَيْهِ وَلَمْ أُسَلِّمْ، فَقَالَ النَّبيُّ - ﷺ: «ارْجِعْ فَقُلْ: السَّلاَمُ عَلَيْكُمْ، أَأَدْخُل؟». رواه أَبُو داود والترمذي، وقال: «حديث حسن».
Kildah ibn al-Ḥanbal (may Allah be pleased with him) reported: I came to the Prophet ﷺ and entered upon him without giving the greeting of peace, so the Prophet ﷺ said: "Go back and say: 'Peace be upon you—may I enter?'" Narrated by Abū Dāwūd and al-Tirmidhī, who said: "A good (ḥasan) hadith."