Hadith 17:666
Yazid ibn Harun narrated to us, he said: Dawud ibn Abi Hind informed us, from Abu Nadra, from Abu Sa’id, that Abu Musa sought permission to enter upon Umar three times, but he did not grant him permission, so he turned away. Umar sent for him and said, "What turned you back?" He said, "I sought permission with the permission-seeking that the Messenger of Allah (peace and blessings of Allah be upon him) commanded us: three times. If permission is granted to us, we enter, and if permission is not granted to us, we return." Umar said, "You must bring me proof for this, or I will do such and such and such and such." So he went to the gathering of his people and asked them, and they testified for him, so he let him go.
حدثنا يزيد بن هارون قال أخبرنا داود بن أبي هند عن أبي نضرة عن أبي سعيد أن أبا موسى استأذن على عمر ثلاثا فلم يأذن له قال فانصرف فأرسل إليه عمر ما ردك قال استأذنت الاستئذان الذي أمرنا به رسول الله صلى الله عليه وسلم ثلاثا فإن أذن لنا دخلنا وأن لم يؤذن لنا رجعنا قال لتأتيني على هذا ببينة أو لأفعلن وأفعلن فأتى مجلس قومه فناشدهم فشهدوا له فخلى عنه