Sahih Muslim 2154 a
Abu Musa Ash'ari reported that he went to 'Umar b. Khattab and greeted him by saying:
As-Salamu-'Alaikum, here is 'Abdullah b. Qais, but he did not permit him (to get in). He (Abu Musa Ash'ari) again greeted him with as-Salamu-'Alaikum and said: Here is Abu Musa, but he (Hadrat 'Umar) did not permit him (to get in). He again said: As-Salam-u-'Alaikum, (and said) here is Ash'ari, (then receiving no response he came back). He (Hadrat 'Umar) said: Bring him back to me, bring him back to me So he went there (in the presence of Hadrat 'Umar) and he said to him: Abu Musa, what made you go back, while we were busy in some work? He said: I heard Allah's Messenger (may. peace be upon him) as saying: Permission should be sought thrice. And if you are permitted, (then get in), otherwise go back. He said: Bring witness to this fact, otherwise I shall do this and that, i. e. I shall punish you. Abu Musa went away and 'Umar said to him (on his departure): It he (Abu Musa) finds a witness he should meet him by the side of the pulpit in the evening and it he does not find a witness you would not find him there. When it was evening he (Hadrat 'Umar) found him (Abu Musa) there. He (Hadrat 'Umar) said: Abu Musa, have you been able to find a witness to what you have said? He said: Yes. Here is Ubayy bin Ka'b, whereupon he (Hadrat 'Umar) said: Yes, he is an authentic (witness). He (Hadrat 'Umar) said: Abu Tufail (the kunya of Ubayy b. Ka'b), what does he (Abu Musa say? Thereupon he said: Ibn Khattab, I heard Allah's Messenger (ﷺ) as saying so. Do not prove to be a hard (task-master) for the Companions of Allah's Messenger (ﷺ), whereupon he Hadrat 'Umar said: Hallowed be Allah. I had heard something (in this connection), but I wished it to be established (as an undeniable fact).
حَدَّثَنَا حُسَيْنُ بْنُ حُرَيْثٍ أَبُو عَمَّارٍ، حَدَّثَنَا الْفَضْلُ بْنُ مُوسَى، أَخْبَرَنَا طَلْحَةُ بْنُ،
 يَحْيَى عَنْ أَبِي بُرْدَةَ، عَنْ أَبِي مُوسَى الأَشْعَرِيِّ، قَالَ جَاءَ أَبُو مُوسَى إِلَى عُمَرَ بْنِ الْخَطَّابِ
 فَقَالَ السَّلاَمُ عَلَيْكُمْ هَذَا عَبْدُ اللَّهِ بْنُ قَيْسٍ ‏.‏ فَلَمْ يَأْذَنْ لَهُ فَقَالَ السَّلاَمُ عَلَيْكُمْ هَذَا أَبُو مُوسَى
 السَّلاَمُ عَلَيْكُمْ هَذَا الأَشْعَرِيُّ ‏.‏ ثُمَّ انْصَرَفَ فَقَالَ رُدُّوا عَلَىَّ رُدُّوا عَلَىَّ ‏.‏ فَجَاءَ فَقَالَ يَا أَبَا
 مُوسَى مَا رَدَّكَ كُنَّا فِي شُغْلٍ ‏.‏ قَالَ سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ ‏
"‏ الاِسْتِئْذَانُ
 ثَلاَثٌ فَإِنْ أُذِنَ لَكَ وَإِلاَّ فَارْجِعْ ‏"‏ ‏.‏ قَالَ لَتَأْتِيَنِّي عَلَى هَذَا بِبَيِّنَةٍ وَإِلاَّ فَعَلْتُ وَفَعَلْتُ ‏.‏ فَذَهَبَ
 أَبُو مُوسَى قَالَ عُمَرُ إِنْ وَجَدَ بَيِّنَةً تَجِدُوهُ عِنْدَ الْمِنْبَرِ عَشِيَّةً وَإِنْ لَمْ يَجِدْ بَيِّنَةً فَلَمْ تَجِدُوهُ
 ‏.‏ فَلَمَّا أَنْ جَاءَ بِالْعَشِيِّ وَجَدُوهُ قَالَ يَا أَبَا مُوسَى مَا تَقُولُ أَقَدْ وَجَدْتَ قَالَ نَعَمْ أُبَىَّ بْنَ
 كَعْبٍ ‏.‏ قَالَ عَدْلٌ ‏.‏ قَالَ يَا أَبَا الطُّفَيْلِ مَا يَقُولُ هَذَا قَالَ سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه
 وسلم يَقُولُ ذَلِكَ يَا ابْنَ الْخَطَّابِ فَلاَ تَكُونَنَّ عَذَابًا عَلَى أَصْحَابِ رَسُولِ اللَّهِ صلى الله عليه
 وسلم ‏.‏ قَالَ سُبْحَانَ اللَّهِ إِنَّمَا سَمِعْتُ شَيْئًا فَأَحْبَبْتُ أَنْ أَتَثَبَّتَ ‏.‏