Muwatta Imam Malik Book of General Subjects, Hadith 3

Malik related to me from Rabia ibn Abi Abd ar-Rahman from another of the ulama of that time that Abu Musa al-Ashari came and asked permission from Umar ibn al-Khattab to enter. He asked permission three times, and then went away Umar ibn al-Khattab sent after him and said, "What's wrong with you? Why didn't you come in?" Abu Musa said, "I heard the Messenger of Allah ﷺ say, 'Ask permission to enter three times. If you are given permission, then enter. If not, go away.' ''Umar said, "Who can confirm this? If you do not bring me someone to confirm it, I will do such-and-such to you."
Abu Musa went out until he came to an assembly 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 ﷺ 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 with me.' " They said to Abu Said al-Khudri, "Go with him". Abu Said was the youngest of them. He went with him and told Umar ibn al-Khattab about that."
Umar ibn al-Khattab said to Abu Musa, "I did not suspect you, but I feared lest people forge sayings of the Messenger of Allah, may Allah bless him and grant him peace."

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 ﷺ 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 ﷺ 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 ﷺ .