It was narrated from Abu Sa'eed Al-Khudri that Abu Musa asked permission to enter upon 'Urnar three limes, 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 .AlIah. concerning that, and they did so, so he let him go. (Sahih)