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 upon Umar three times, but I was not granted permission, so I went back."Umar asked him, "What kept you from coming in?"Abu Musa replied, "I sought permission three times and was not granted it, so I returned. The Messenger of Allah ﷺ said, 'When one of you seeks permission three times and is not granted it, let him go back.'"Umar said, "By Allah, you must bring me proof of this. Is there anyone among you who heard it from the Prophet ﷺ?"Ubayy bin Kaab said, "By Allah, none will go with you except the youngest of the people."I was the youngest of the group, so I went with him and informed Umar that the Prophet ﷺ had indeed said that.
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.
Bukhari narrated it in the Book of Seeking Permission (6245), and this is his wording, and Muslim in the Book of Manners (2153), both of them through Sufyan bin Uyayna, who said, "Yazid bin Khusayfa narrated to us, by Allah," from Busr bin Saeed, who said, "I heard Abu Saeed Khudri say," and then he related it.