In The Name of Allaah, The Most Merciful, The Bestower of Mercy
Abu Sa’eed al-Khudri reported: ‘’While I was in one the gatherings of the Ansar, Abu Musa came as if he was scared, and said, ‘I asked permission to enter upon ‘Umar three times and he did not give me permission, so I returned. (When Umar came to know about it) he said to Abu Musa, ‘’Why did you not enter?” Abu Musa said, ‘’I asked permission to enter three times and was not given permission, so I went back, for the Messenger of Allah (sallal-laahu-alayhi-wasallam) said, ‘’When one of you asks for permission to enter three times and is not given it, he should go back.” He (Umar) said, ‘’By Allah, you will bring witnesses for it.” (Abu Musa went to a gathering of the Ansaar) and said: ‘’Is there any of you who heard it from the Prophet, (sallal-laahu-alayhi-wasallam)?” Ubayy ibn Ka’b said, ‘’By Allah, only the youngest of the people will go with you.’’ I was the youngest of the people and I went with him and informed Umar that the Prophet (sallal-laahu-alayhi-wasallam) had said that. [Bukhaari 8/6245]
Imaam Abdul Azeez Bin Baaz [rahimahullaah] said: This [i.e. Umar asking for a witness] was for the purpose of verification because it is [related] to Ibaadaat [acts of worship].
[Source: Al-Hulalul Ibreeziyyah Min At-Ta’liqaat Al-Baaziyyah Alaa saheeh Al-Bukhaari 4/185. abridged]