Sahih Bukhari: Hadith 6023

While I was present in one of the gatherings of the Ansar, Abu Musa came as if he was scared, and said, "I asked permission to enter upon `Umar three times, but I was not given the permission, so I returned." (When `Umar came to know about it) he said to Abu Musa, "Why did you not enter?'. Abu Musa replied, "I asked permission three times, and I was not given it, so I returned, for Allah's Messenger (ﷺ) said, "If anyone of you asks the permission to enter thrice, and the permission is not given, then he should return.' " `Umar said, "By Allah! We will ask Abu Musa to bring witnesses for it." (Abu Musa went to a gathering of the Ansar and said). "Did anyone of you hear this from the Prophet (ﷺ) ?" Ubai bin Ka`b said, "By Allah, none will go with you but the youngest of the people (as a witness)." (Abu Sa`id) was the youngest of them, so I went with Abu Musa and informed `Umar that the Prophet (ﷺ) had said so. (See Hadith No. 277, Vol. 3)