Seeking permission to enter

It is narrated from Sayyiduna Abu Musa al-Ash'ari (may Allah be pleased with him) that a man asked him about seeking permission when entering (the room) of his parents, and he replied: "Yes, you must certainly ask permission. Would you like to see some private state or nakedness of theirs that ought to be covered?"
This athar (report) is sahih, and Imam Ibn Abi Shaybah recorded it in al-Musannaf (hadith: 2645).

It is narrated from Muslim bin Nudhayr (may Allah have mercy on him) that a man asked Sayyiduna Hudhayfah (may Allah be pleased with him): "Should I seek permission even when entering (the room) of my mother?" He said: "Yes! If you do not seek her permission, you may well see her in a state that would be unpleasant and displeasing to you."
This athar (report) is sahih, and Imam Ibn Abi Shaybah recorded it in al-Musannaf (hadith: 2656) and Imam al-Bukhari in al-Adab al-Mufrad (hadith: 1063).

It is narrated from Hilal bin Yasaf (may Allah have mercy on him) that he said: "The Messenger of Allah (peace and blessings be upon him) forbade that a person, when seeking permission (to enter someone's house), should stand directly facing the door."
This narration is mursal, because Hilal bin Yasaf is a Successor (Tabi'i) and has reported directly from the Prophet (peace and blessings be upon him); however, its meaning is established by other authentic hadiths, which stress that when asking permission one should stand to the right or left of the door so that the privacy of the household is not violated.

It is narrated from Sayyidah Umm Umarah (may Allah be pleased with her) that she said: I heard the Messenger of Allah (peace and blessings be upon him) when we were returning from Khaybar and he (peace and blessings be upon him) was staying at the place called al-Jurf; he (peace and blessings be upon him) was saying: "Do not go in to your households (suddenly at night) after the Isha prayer."
Imam al-Harith bin Abi Usamah recorded this hadith in his Musnad (Bughyat al-Bahith: 442); the narrator in its chain, Muhammad bin Umar al-Waqidi, is weak according to the majority, although the basis of the prohibition of arriving home suddenly at night is established by other reliable hadiths of Sahih al-Bukhari and Muslim.

It is narrated from Sayyiduna Anas (may Allah be pleased with him) that he said: "The doors (of the blessed chambers) of the Messenger of Allah (peace and blessings be upon him) used to be knocked on with fingernails (because of the Companions' utmost courtesy and respect)."
Imam Abu Ya'la recorded this hadith in his Musnad; this narration shows the etiquette of the court of the Messenger (peace and blessings be upon him) and the Companions' practice of utmost reverence and of presenting themselves quietly.