وَحَدَّثَنِي مَالِكٌ، عَنْ رَبِيعَةَ بْنِ أَبِي عَبْدِ الرَّحْمَنِ، عَنْ غَيْرِ، وَاحِدٍ، مِنْ عُلَمَائِهِمْ أَنَّ أَبَا مُوسَى الأَشْعَرِيَّ، جَاءَ يَسْتَأْذِنُ عَلَى عُمَرَ بْنِ الْخَطَّابِ فَاسْتَأْذَنَ ثَلاَثًا ثُمَّ رَجَعَ فَأَرْسَلَ عُمَرُ بْنُ الْخَطَّابِ فِي أَثَرِهِ فَقَالَ مَا لَكَ لَمْ تَدْخُلْ فَقَالَ أَبُو مُوسَى سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ ‏"‏ الاِسْتِئْذَانُ ثَلاَثٌ فَإِنْ أُذِنَ لَكَ فَادْخُلْ وَإِلاَّ فَارْجِعْ ‏"‏ ‏.‏ فَقَالَ عُمَرُ وَمَنْ يَعْلَمُ هَذَا لَئِنْ لَمْ تَأْتِنِي بِمَنْ يَعْلَمُ ذَلِكَ لأَفْعَلَنَّ بِكَ كَذَا وَكَذَا ‏.‏ فَخَرَجَ أَبُو مُوسَى حَتَّى جَاءَ مَجْلِسًا فِي الْمَسْجِدِ يُقَالُ لَهُ مَجْلِسُ الأَنْصَارِ فَقَالَ إِنِّي أَخْبَرْتُ عُمَرَ بْنَ الْخَطَّابِ أَنِّي سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ ‏"‏ الاِسْتِئْذَانُ ثَلاَثٌ فَإِنْ أُذِنَ لَكَ فَادْخُلْ وَإِلاَّ فَارْجِعْ ‏"‏ ‏.‏ فَقَالَ لَئِنْ لَمْ تَأْتِنِي بِمَنْ يَعْلَمُ هَذَا لأَفْعَلَنَّ بِكَ كَذَا وَكَذَا ‏.‏ فَإِنْ كَانَ سَمِعَ ذَلِكَ أَحَدٌ مِنْكُمْ فَلْيَقُمْ مَعِي ‏.‏ فَقَالُوا لأَبِي سَعِيدٍ الْخُدْرِيِّ قُمْ مَعَهُ ‏.‏ وَكَانَ أَبُو سَعِيدٍ أَصْغَرَهُمْ فَقَامَ مَعَهُ فَأَخْبَرَ بِذَلِكَ عُمَرَ بْنَ الْخَطَّابِ ‏.‏ فَقَالَ عُمَرُ بْنُ الْخَطَّابِ لأَبِي مُوسَى أَمَا إِنِّي لَمْ أَتَّهِمْكَ وَلَكِنْ خَشِيتُ أَنْ يَتَقَوَّلَ النَّاسُ عَلَى رَسُولِ اللَّهِ صلى الله عليه وسلم ‏.‏

Malik related to me from Rabia ibn Abi Abd ar-Rahman from anotherof the ulama of that time that Abu Musa al-Ashari came and askedpermission from Umar ibn al-Khattab to enter. He asked permissionthree times, and then went away Umar ibn al-Khattab sent after him andsaid, "What's wrong with you? Why didn't you come in?" Abu Musa said,"I heard the Messenger of Allah, may Allah bless him and grant himpeace, say, 'Ask permission to enter three times. If you are givenpermission, then enter. If not, go away.' ''Umar said, "Who canconfirm this? If you do not bring me someone to confirm it, I will dosuch-and-such to you." Abu Musa went out until he came to anassembly in the mosque which was called the Majlis-al-Ansar. He said,"I told Umar ibn al-Khattab that I heard the Messenger of Allah, mayAllah bless him and grant him peace, say, 'Ask permission three times.If you are given permission, then enter. If not, go away.' Umar said,'If you do not bring me someone who can confirm it, I will do such-and-such to you'. If any of you have heard that, let him come withme.' " They said to Abu Said al-Khudri, "Go with him". Abu Said wasthe youngest of them. He went with him and told Umar ibn al-Khattababout that." Umar ibn al-Khattab said to Abu Musa, "I did notsuspect you, but I feared lest people forge sayings of the Messengerof Allah, may Allah bless him and grant him peace." Translation Not Available

Book reference: Muwatta Imam Malik Book 54 Hadith 1768
Web reference: Muwatta Imam Malik Book 54 Hadith 3