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 ʿUmar three times, but I was not given the permission, so I returned." (When ʿUmar 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 Messenger of Allah ﷺ said, "If anyone of you asks the permission to enter thrice, and the permission is not given, then he should return.' " ʿUmar 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 Kaʿb said, "By Allah, none will go with you but the youngest of the people (as a witness)." (Abu Saʿid) was the youngest of them, so I went with Abu Musa and informed ʿUmar that the Prophet ﷺ had said so. (See Hadith No. 277, Vol. 3)  

كُنْتُ فِي مَجْلِسٍ مِنْ مَجَالِسِ الأَنْصَارِ إِذْ جَاءَ أَبُو مُوسَى كَأَنَّهُ مَذْعُورٌ فَقَالَ اسْتَأْذَنْتُ عَلَى عُمَرَ ثَلاَثًا فَلَمْ يُؤْذَنْ لِي فَرَجَعْتُ فَقَالَ مَا مَنَعَكَ قُلْتُ اسْتَأْذَنْتُ ثَلاَثًا فَلَمْ يُؤْذَنْ لِي فَرَجَعْتُ وَقَالَ رَسُولُ اللَّهِ ﷺ إِذَا اسْتَأْذَنَ أَحَدُكُمْ ثَلاَثًا فَلَمْ يُؤْذَنْ لَهُ فَلْيَرْجِعْ فَقَالَ وَاللَّهِ لَتُقِيمَنَّ عَلَيْهِ بِبَيِّنَةٍ أَمِنْكُمْ أَحَدٌ سَمِعَهُ مِنَ النَّبِيِّ ﷺ فَقَالَ أُبَىُّ بْنُ كَعْبٍ وَاللَّهِ لاَ يَقُومُ مَعَكَ إِلاَّ أَصْغَرُ الْقَوْمِ فَكُنْتُ أَصْغَرَ الْقَوْمِ فَقُمْتُ مَعَهُ فَأَخْبَرْتُ عُمَرَ أَنَّ النَّبِيَّ ﷺ قَالَ ذَلِكَ وَقَالَ ابْنُ الْمُبَارَكِ أَخْبَرَنِي ابْنُ عُيَيْنَةَ حَدَّثَنِي يَزِيدُ عَنْ بُسْرٍ سَمِعْتُ أَبَا سَعِيدٍ بِهَذَا