Tirmidhi

1:
From this it is understood that sometimes a lesser person may know a matter of religious knowledge that a great scholar does not know. For example, Umar radi Allahu anhu did not know that after seeking permission three times, if permission is not granted, one should return.

Hadith Commentary:
Benefits and Issues:
From this hadith, it is understood that Umar (radi Allahu anhu) demanded evidence (bayyinah) from Abu Musa (radi Allahu anhu) on the following day.
Since from other ahadith it is known that Umar (radi Allahu anhu) summoned Abu Musa al-Ash'ari (radi Allahu anhu) on that same day and, upon his response, demanded evidence, the answer to this is that Umar (radi Allahu anhu), after becoming free from his engagements,
inquired about him, and when he was informed that he had come and left, he sent a messenger to him,
but he could not be found, and so he himself appeared the next day.

Benefits and Issues:
➊
The original Islamic etiquette for seeking permission is to say "As-salamu alaykum" and to seek permission by saying salam three times.
Knocking or ringing the bell should be analogized to this.
To exceed this is contrary to etiquette; it is possible that the person in the house is engaged in some specific task or is resting, so he should not be disturbed.
Furthermore, when knocking or ringing the bell, one should not display disrespect or rudeness.
It has been observed that some people knock so loudly that even the peace of the neighbors is disturbed.
Similarly, some people ring the bell so continuously as if they intend to raise the dead or make the deaf hear; these are signs of our moral decline.

➋ If permission or a response is not received, one should not become unnecessarily upset, but rather should return.

➌ The reason why Umar (radi Allahu anhu) was so strict in narrating the ahadith of the Messenger (sallallahu alayhi wa sallam) was that people might not begin to attribute things to the Messenger (sallallahu alayhi wa sallam) without certainty and trust. And may Allah reward him with goodness—this strictness of his was a great wisdom.

Benefits and Issues:

➊
It is prohibited to enter someone’s house without permission.

➋
The method of seeking permission is as follows:
“As-salamu alaykum, may I come in?” (Sunan Abi Dawud, Chapter: How to Seek Permission, Hadith: 5177)

➌
If, upon seeking permission once, there is no response, then one should seek permission a second and third time.
Nowadays, the method of seeking permission has changed, such as ringing the bell; this too should only be done three times at intervals.
If there is no response, one should leave.
One should not disturb the entire neighborhood by repeatedly ringing the bell.

➍
If, after seeking permission three times, permission is still not granted, one should return without being upset with the household.
It is possible that the owner of the house is not present, or there may be a reasonable excuse for not granting permission.

➎
Umar (radi Allahu anhu) requested witnesses because he wanted further assurance.
Another purpose was that when people see that Umar (radi Allahu anhu) is cautious even regarding the hadith of the Messenger of Allah (sallallahu alayhi wa sallam) and does not accept it from irresponsible people, then no one will dare to narrate hadiths without thorough investigation.
In this way, irresponsible people will not narrate hadiths with incorrect wording or fabricate them from themselves.

➏
Hadith is the foundation of the religion; therefore, distinguishing between authentic (sahih) and weak (da’if) hadith is very important.

➐
Weak (da’if) hadiths are present in the books of hadith.
The reason for this is that the Imams of hadith used to mention the chain of narration (isnad) with every hadith, so that the listeners would know the status of the hadith.
Sometimes, they would narrate a weak hadith because it was possible that another chain of narration might be found for it, which would remove its weakness and make the hadith acceptable. However, not every hadith narrated through multiple chains becomes acceptable; rather, certain conditions must be met, the details of which are found in the books of the principles of hadith (usul al-hadith).