Musnad Ahmad 788:127

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Abu Maslamah, from Abu Nadrah, from Abu Sa‘id al-Khudri, who said: Abu Musa sought permission to enter upon ‘Umar, may Allah be pleased with them both. He said: "One, two, three [times]," then Abu Musa turned back. ‘Umar, may Allah be pleased with him, said to him: "You shall bring proof for this, or I shall certainly do [something to you]." It was as if he was saying: "I will make you an example in the horizons." Abu Musa went to a gathering in which there were Ansar and mentioned that to them. He said: "Do you not know that the Messenger of Allah, may Allah bless him and grant him peace, said: 'If one of you seeks permission three times and is not granted permission, then let him return'?" They said: "Yes. No one shall stand with you except the youngest of us." Abu Sa‘id al-Khudri stood up and went to ‘Umar, may Allah be pleased with him, and said: "This is Abu Sa‘id," so he let him go.