Abu Saeed Khudri reported,

I was sitting in one of the gatherings of the Ansar when Abu Musa came in looking alarmed. He said, "I sought permission to enter Umar's house three times, but I was not given permission, so I left."Umar later said to him, "What kept you from coming in?"Abu Musa replied, "I sought permission three times and was not granted it, so I left. The Messenger of Allah ﷺ said, 'When one of you seeks permission three times and is not given permission, let him return.'"Umar said, "By Allah, you must produce evidence for this. Is there anyone among you who heard it from the Prophet ﷺ?"Ubayy bin Kaab said, "By Allah, none will stand with you except the youngest of the people."I was the youngest of the group, so I stood with him and informed Umar that the Prophet ﷺ had indeed said that.In another narration, the narrator said, We were in a gathering at the home of Ubayy bin Kaab when Abu Musa Ashari came in angry and stood there. He said, "I ask you by Allah, has any of you heard the Messenger of Allah ﷺ say, 'Seeking permission is three times. If you are granted entry, enter, otherwise return'?"Ubayy asked, "What is the matter?"Abu Musa said, "Yesterday I sought permission to enter Umar bin Khattab's home three times. I was not granted permission, so I left. Then I came to him today and entered, and I informed him that I had come yesterday, given the greeting three times, and then departed."Umar said, "We heard you, but we were busy at the time. Why did you not wait until you were granted permission?"Abu Musa replied, "I sought permission as I heard the Messenger of Allah ﷺ instruct."Umar said, "By Allah, I will surely strike your back and stomach, or you will bring me someone who can testify to this."Ubayy bin Kaab said, "By Allah, none will stand with you except the youngest of us. Stand up, Abu Saeed."So I stood and went with him to Umar, and I said, "I heard the Messenger of Allah ﷺ say this."

عن أبي سعيد الخدري، قال

كنت في مجلس من مجالس الأنصار، إذ جاء أبو موسى كأنه مذعور، فقال: استأذنت على عمر ثلاثا، فلم يؤذن لي فرجعت، فقال: ما منعك؟ قلت: استأذنت ثلاثا فلم يؤذن لي فرجعت، وقال رسول الله ﷺ: إذا استأذن أحدكم ثلاثا فلم يؤذن له فليرجع فقال: والله لتقيمن عليه ببينة، أمنكم أحد سمعه من النبي ﷺ فقال أبي بن كعب: والله لا يقوم معك إلا أصغر القوم، فكنت أصغر القوم فقمت معه، فأخبرت عمر أن النبي ﷺ قال ذلك. وفي رواية: كنا في مجلس عند أبي بن كعب، فأتى أبو موسى الأشعري مغضبا حتى وقف، فقال: أنشدكم الله هل سمع أحد منكم رسول الله ﷺ يقول: الاستئذان ثلاث، فإن أذن لك، وإلا فارجع قال أبي: وما ذاك؟ قال: استأذنت على عمر بن الخطاب أمس ثلاث مرات، فلم يؤذن لي فرجعت، ثم جئته اليوم فدخلت عليه، فأخبرته، أني جئت أمس فسلمت ثلاثا، ثم انصرفت قال: قد سمعناك ونحن حينئذ على شغل، فلو ما استأذنت حتى يؤذن لك قال: استأذنت كما سمعت رسول الله ﷺ قال: فوالله، لأوجعن ظهرك وبطنك، أو لتأتين بمن يشهد لك على هذا، فقال أبي بن كعب: فوالله، لا يقوم معك إلا أحدثنا سنا، قم، يا أبا سعيد، فقمت حتى أتيت عمر، فقلت: قد سمعت رسول الله ﷺ يقول هذا.