Sahih Muslim Book 25, Hadith Number 5354.

Chapter : Asking permission to enter a house.

Abu Sa’id Khudri reported: I wins sitting in Medina in the company of the Ansar that Abu Musa came trembling with fear. We said to him: What is the matter? He said: ‘Umar (Allah be pleated with him) sent for me. I went to him and paid him salutation thrice at (his) door but he made no response to me and so I came back Thereupon he (‘Umar) said: What stood in your way that you did not turn up? I said: I did come to you and paid you salutations at your door three times but I was not given any response, so I came back as the Messenger of Allah (may peace be upon him) has said: When any one of you seeks permission three times and he i.e. not granted permission, he should come back. Umar said: Bring a witness to support that you say, otherwise I shall take you to task. Ubayy b. Ka’b said: None should stand with him (as a witness) but the youngest amongst the people. Abu Sa’id said: I am the youngest amongst the people, whereupon he said: Then you go with him (to support his contention).