Sahih Muslim 2154a

Abu Musa Ash'ari reported that he went to 'Umar b. Khattab and greeted him by saying:As-Salamu-'Alaikum, here is 'Abdullah b. Qais, but he did not permit him (to get in). He (Abu Musa Ash'ari) again greeted him with as-Salamu-'Alaikum and said: Here is Abu Musa, but he (Hadrat 'Umar) did not permit him (to get in). He again said: As-Salam-u-'Alaikum, (and said) here is Ash'ari, (then receiving no response he came back). He (Hadrat 'Umar) said: Bring him back to me, bring him back to me So he went there (in the presence of Hadrat 'Umar) and he said to him: Abu Musa, what made you go back, while we were busy in some work? He said: I heard Allah's Messenger (may. peace be upon him) as saying: Permission should be sought thrice. And if you are permitted, (then get in), otherwise go back. He said: Bring witness to this fact, otherwise I shall do this and that, i. e. I shall punish you. Abu Musa went away and 'Umar said to him (on his departure): It he (Abu Musa) finds a witness he should meet him by the side of the pulpit in the evening and it he does not find a witness you would not find him there. When it was evening he (Hadrat 'Umar) found him (Abu Musa) there. He (Hadrat 'Umar) said: Abu Musa, have you been able to find a witness to what you have said? He said: Yes. Here is Ubayy bin Ka'b, whereupon he (Hadrat 'Umar) said: Yes, he is an authentic (witness). He (Hadrat 'Umar) said: Abu Tufail (the kunya of Ubayy b. Ka'b), what does he (Abu Musa say? Thereupon he said: Ibn Khattab, I heard Allah's Messenger (ﷺ) as saying so. Do not prove to be a hard (task-master) for the Companions of Allah's Messenger (ﷺ), whereupon he Hadrat 'Umar said: Hallowed be Allah. I had heard something (in this connection), but I wished it to be established (as an undeniable fact)