so the Prophet gave him permission and the man entered. Reported by Abū Dāwūd with a sound chain.

873 – Kalladah ibn Ḥunbul (may Allah be pleased with him) said: “I came to the Prophet (peace and blessings be upon him), entered upon him, and did not greet him with peace. The Prophet (peace and blessings be upon him) said, ‘Go back and say, Peace be upon you; may I enter?’” Reported by Abū Dāwūd and al-Tirmidhī, who graded it as hasan (sound).

Explanation
The author—may Allah have mercy on him—opens his book Riyāḍ al-Ṣāliḥīn with the chapter on seeking permission and its etiquette. Seeking permission means requesting leave: you ask the homeowner to allow you to come in. If he grants you leave, then enter; if he does not, then do not enter—even if he clearly says to you, “Go back,” then go back, just as Allah the Exalted says, “And if you are told, ‘Go back,’ then go back; that is purer for you.”

As for you, O homeowner, do not feel shy to say, “Go back,” and you, O visitor, do not become angry if he tells you, “Go back,” for a person may have a need or may simply not be ready to receive guests. You must not coerce him or place him in an awkward position. When you turn back after he has said, “Go back,” Allah affirms that this is purer for you—“Go back; that is purer for you,” that is, purer and more wholesome for your hearts.