Musnad Ahmad 788:193

Yazid narrated to us, Dawud informed us, from Abu Nadra, from Abu Sa’id al-Khudri, who said: Abu Musa sought permission to enter upon Umar, may Allah be pleased with them both, three times, but he was not granted permission, so he returned. Umar met him and said: "What is the matter with you that you returned?" He said: "I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'Whoever seeks permission three times and is not granted permission, let him return.'" Umar said: "You shall bring proof for this, or I will do such and such to you." So he went to the gathering of his people and adjured them by Allah the Exalted. I (Abu Sa’id) said: "I am with you." So they testified to that for him, and he (Umar) let him go.