Sunan Ibn Majah Book 33, Hadith 3706

حَدَّثَنَا أَبُو بَكْرٍ، حَدَّثَنَا يَزِيدُ بْنُ هَارُونَ، أَنْبَأَنَا دَاوُدُ بْنُ أَبِي هِنْدٍ، عَنْ أَبِي نَضْرَةَ، عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، أَنَّ أَبَا مُوسَى، اسْتَأْذَنَ عَلَى عُمَرَ ثَلاَثًا فَلَمْ يُؤْذَنْ لَهُ فَانْصَرَفَ فَأَرْسَلَ إِلَيْهِ عُمَرُ مَا رَدَّكَ قَالَ اسْتَأْذَنْتُ الاِسْتِئْذَانَ الَّذِي أَمَرَنَا بِهِ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ ثَلاَثًا فَإِنْ أُذِنَ لَنَا دَخَلْنَا وَإِنْ لَمْ يُؤْذَنْ لَنَا رَجَعْنَا ‏.‏ قَالَ فَقَالَ لَتَأْتِيَنِّي عَلَى هَذَا بِبَيِّنَةٍ أَوْ لأَفْعَلَنَّ ‏.‏ فَأَتَى مَجْلِسَ قَوْمِهِ فَنَاشَدَهُمْ فَشَهِدُوا لَهُ فَخَلَّى سَبِيلَهُ ‏.‏

It was narrated from Abu Saeed Khudri that Abu Musa asked permission to enter upon 'Umar three times, and he did not give him permission, so he went away.'Umar sent word to him saying:"Why did you go back?" He said: "I asked permission to enter three times, as the Messenger of Allah(ﷺ) enjoined upon us, then if we are given permission we should enter, otherwise we should go back." He said: "You should bring me proof of that, or else!" Then he came to a gathering of his people and asked them to swear by Allah concerning that, and they did so, so he let him go