Abu Musa came to Umar's door and asked permission to enter.Umar said, "That is once."Then he asked permission a second time, and Umar said, "That is twice."Then he asked permission a third time, and Umar said, "That is three times."Then Abu Musa left. Umar sent someone after him and brought him back, saying, "If this is something you memorized from the Messenger of Allah ﷺ, then produce it. Otherwise, I will make an example of you."Abu Saeed said, Abu Musa came to us and said, "Do you not know that the Messenger of Allah ﷺ said, 'Seeking permission is three times'?"They began to laugh.I said, "Your Muslim brother has come to you frightened, and you laugh? Go on, I am your partner in this punishment."So he went to Umar and said, "This is Abu Saeed."
أن أبا موسى أتى باب عمر، فاستأذن، فقال عمر: واحدة، ثم استأذن الثانية، فقال عمر: ثنتان، ثم استأذن الثالثة، فقال عمر: ثلاث، ثم انصرف فأتبعه فرده، فقال: إن كان هذا شيئا حفظته من رسول الله ﷺ فهاه، وإلا، فلأجعلنك عظة، قال أبو سعيد: فأتانا فقال: ألم تعلموا أن رسول الله ﷺ قال: الاستئذان ثلاث؟ قال: فجعلوا يضحكون، قال فقلت: أتاكم أخوكم المسلم قد أفزع، تضحكون؟ انطلق فأنا شريكك في هذه العقوبة، فأتاه فقال: هذا أبو سعيد.
Muslim narrated it in the book of manners (2153:35), from Nasr bin Ali Jahdami, who said, "Bishr, meaning Ibn Mufaddal, told us, Saeed bin Yazid told us, from Abu Nadra, from Abu Saeed," and he mentioned it.