Al-Muwatta Hadith 54.3

Malik related to me from Rabia ibn Abi Abd ar-Rahman from another of the ulama of that time that Abu Musa al-Ashari came and asked permission from Umar ibn al-Khattab to enter. He asked permission three times, and then went away Umar ibn al-Khattab sent after him and said, "What's wrong with you? Why didn't you come in?" Abu Musa said, "I heard the Messenger of Allah, may Allah bless him and grant him peace, say, 'Ask permission to enter three times. If you are given permission, then enter. If not, go away.' ''Umar said, "Who can confirm this? If you do not bring me someone to confirm it, I will do such-and-such to you." Abu Musa went out until he came to an assembly in the mosque which was called the Majlis-al-Ansar. He said, "I told Umar ibn al-Khattab that I heard the Messenger of Allah, may Allah bless him and grant him peace, say, 'Ask permission three times. If you are given permission, then enter. If not, go away.' Umar said, 'If you do not bring me someone who can confirm it, I will do such-and-such to you'. If any of you have heard that, let him come with me.' " They said to Abu Said al-Khudri, "Go with him". Abu Said was the youngest of them. He went with him and told Umar ibn al-Khattab about that." Umar ibn al-Khattab said to Abu Musa, "I did not suspect you, but I feared lest people forge sayings of the Messenger of Allah, may Allah bless him and grant him peace."