"Abu Musa sought permission to enter upon 'Umar. He said: 'As-Salamu 'Alaykum (Peace be upon you). May I enter?' 'Umar said: 'Once.' Then he was silent for some time. Then he said: 'As-Salamu 'Alaykum (Peace be upon you). May I enter?' 'Umar said: Twice.' Then he was silent for some time. Then he said: 'As-salamu 'Alaykum (Peace be upon you). May I enter?' 'Umar said: 'Three times.' Then he (Abu Musa) left. 'Umar said to the gate-keeper: 'What did he do?' He replied: 'He left." He said: 'Bring him to me.' So when he came, 'Umar said to him: 'What is this that you have done?' He said: 'The Sunnah.' He said: 'The Sunnah? By Allah! You had better bring me proof or a witness to clarify this, or I will do this or that to you.'" He said "So he came to us while we were sitting with Ansar. He said: 'O People of the Ansar! Are you not the most knowledgeable people about the Ahadith of the Messenger of Allah ﷺ? Did the Messenger of Allah ﷺ not say: "Seeking permission is to be done three time. Either you are permitted, or other wise leave?" The people began joking. Abu Sa'eed said: "Then I raised my head toward him and said: "Whatever punishment you are afflicted with because of this, then I shall be your partner in it." So he went to 'Umar to inform him him about it, and 'Umar said: "I did not know about about this."  

عُمَرُ وَاحِدَةٌ ثُمَّ سَكَتَ سَاعَةً ثُمَّ قَالَ السَّلاَمُ عَلَيْكُمْ أَأَدْخُلُ قَالَ عُمَرُ ثِنْتَانِ ثُمَّ سَكَتَ سَاعَةً فَقَالَ السَّلاَمُ عَلَيْكُمْ أَأَدْخُلُ فَقَالَ عُمَرُ ثَلاَثٌ ثُمَّ رَجَعَ فَقَالَ عُمَرُ لِلْبَوَّابِ مَا صَنَعَ قَالَ رَجَعَ قَالَ عَلَىَّ بِهِ فَلَمَّا جَاءَهُ قَالَ مَا هَذَا الَّذِي صَنَعْتَ قَالَ السُّنَّةَ قَالَ السُّنَّةَ وَاللَّهِ لَتَأْتِيَنِّي عَلَى هَذَا بِبُرْهَانٍ أَوْ بِبَيِّنَةٍ أَوْ لأَفْعَلَنَّ بِكَ قَالَ فَأَتَانَا وَنَحْنُ رُفْقَةٌ مِنَ الأَنْصَارِ فَقَالَ يَا مَعْشَرَ الأَنْصَارِ أَلَسْتُمْ أَعْلَمَ النَّاسِ بِحَدِيثِ رَسُولِ اللَّهِ ﷺ أَلَمْ يَقُلْ رَسُولُ اللَّهِ ﷺ الاِسْتِئْذَانُ ثَلاَثٌ فَإِنْ أُذِنَ لَكَ وَإِلاَّ فَارْجِعْ فَجَعَلَ الْقَوْمُ يُمَازِحُونَهُ قَالَ أَبُو سَعِيدٍ ثُمَّ رَفَعْتُ رَأْسِي إِلَيْهِ فَقُلْتُ فَمَا أَصَابَكَ فِي هَذَا مِنَ الْعُقُوبَةِ فَأَنَا شَرِيكُكَ قَالَ فَأَتَى عُمَرَ فَأَخْبَرَهُ بِذَلِكَ فَقَالَ عُمَرُ مَا كُنْتُ عَلِمْتُ بِهَذَا وَفِي الْبَابِ عَنْ عَلِيٍّ وَأُمِّ طَارِقٍ مَوْلاَةِ سَعْدٍ  

that Uman bin Al-Khattab said: "I sought permission (to enter) from the Messenger of Allah ﷺ three times, then he permitted me."  

اسْتَأْذَنْتُ عَلَى رَسُولِ اللَّهِ ﷺ ثَلاَثًا فَأَذِنَ لِي