Must a son open the door for his father, even if he is not prepared to receive him, and does the noble verse: "And if it is said to you, 'Go back,' then go back; it is purer for you" apply to this situation?

Children must seek permission from their parents when they arrive and ask to enter, because leaving their request unanswered, when able to do so, is an act of disobedience (ʿuqūq) and contradicts the righteousness (birr) and good treatment (iḥsān) commanded by Allah. Ibn Hajar al-ʿAsqalani views the great importance of honoring one's parents and answering their call. A father's entry into his son's dwelling is akin to his call, and it must be answered, especially if he would be harmed otherwise. The verse: "O you who have believed, do not enter houses other than your own houses until you ascertain intimacy and greet their people" (Quran 24:27) indicates the obligation for the visitor, including the father, to seek permission. However, it does not address the ruling on the homeowner granting permission, so it does not imply that it is permissible to refuse the father permission to enter.