Sahih Muslim 2153 a
Abu Sa'id Khudri reported:
I was sitting in Medina in the company of the Ansar when Abu Musa came trembling with fear. We said to him: What is the matter? He said: 'Umar (Allah be pleased with him) sent for me. I went to him and paid him salutation thrice at (his) door but he made no response to me and so I came back. Thereupon he ('Umar) said: What stood in your way that you did not turn up? I said: I did come to you and paid you salutations at your door three times but I was not given any response, so I came back as the Messenger of Allah (ﷺ) has said: When any one of you seeks permission three times and he is not granted permission, he should come back. Umar said: Bring a witness to support that you say, otherwise I shall take you to task. Ubayy b. Ka'b said: None should stand with him (as a witness) but the youngest amongst the people. Abu Sa'id said: I am the youngest amongst the people, whereupon he said: Then you go with him (to support his contention).
حَدَّثَنِي عَمْرُو بْنُ مُحَمَّدِ بْنِ بُكَيْرٍ النَّاقِدُ، حَدَّثَنَا سُفْيَانُ بْنُ عُيَيْنَةَ، حَدَّثَنَا - وَاللَّهِ،
 - يَزِيدُ بْنُ خُصَيْفَةَ عَنْ بُسْرِ بْنِ سَعِيدٍ، قَالَ سَمِعْتُ أَبَا سَعِيدٍ الْخُدْرِيَّ، يَقُولُ كُنْتُ جَالِسًا
 بِالْمَدِينَةِ فِي مَجْلِسِ الأَنْصَارِ فَأَتَانَا أَبُو مُوسَى فَزِعًا أَوْ مَذْعُورًا ‏.‏ قُلْنَا مَا شَأْنُكَ قَالَ
 إِنَّ عُمَرَ أَرْسَلَ إِلَىَّ أَنْ آتِيَهُ فَأَتَيْتُ بَابَهُ فَسَلَّمْتُ ثَلاَثًا فَلَمْ يَرُدَّ عَلَىَّ فَرَجَعْتُ فَقَالَ مَا مَنَعَكَ
 أَنْ تَأْتِيَنَا فَقُلْتُ إِنِّي أَتَيْتُكَ فَسَلَّمْتُ عَلَى بَابِكَ ثَلاَثًا فَلَمْ يَرُدُّوا عَلَىَّ فَرَجَعْتُ وَقَدْ قَالَ رَسُولُ
 اللَّهِ صلى الله عليه وسلم ‏
"‏ إِذَا اسْتَأْذَنَ أَحَدُكُمْ ثَلاَثًا فَلَمْ يُؤْذَنْ لَهُ فَلْيَرْجِعْ ‏"‏ ‏.‏ فَقَالَ عُمَرُ
 أَقِمْ عَلَيْهِ الْبَيِّنَةَ وَإِلاَّ أَوْجَعْتُكَ ‏.‏ فَقَالَ أُبَىُّ بْنُ كَعْبٍ لاَ يَقُومُ مَعَهُ إِلاَّ أَصْغَرُ الْقَوْمِ ‏.‏ قَالَ أَبُو
 سَعِيدٍ قُلْتُ أَنَا أَصْغَرُ الْقَوْمِ ‏.‏ قَالَ فَاذْهَبْ بِهِ ‏.‏