Musnad Ahmad 31:157

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 three times, but Umar did not grant him 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 must 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 Almighty and Majestic. I said: "I am with you," so they testified to that for him, and he let him go.