Sunan Ibn Majah, Chapter: The Chapters on Etiquette

that Abu Musa asked permission to enter upon Umar three times, and he did not give him permission, so he went away. Umar sent word to him saying: "Why did you go back?" He said: "I asked permission to enter three times, as the Messenger of Allah (saw) enjoined upon us, then if we are given permission we should enter, otherwise we should go back." He said: "You should bring me proof of that, or else!" Then he came to a gathering of his people and asked them to swear by Allah concerning that, and they did so, so he let him go."