Musnad Ahmad 31:44

Sufyan narrated to us, Yazid ibn Khusayfah narrated to us, from Busr ibn Sa’id, from Abu Sa’id al-Khudri, who said: I was in a circle of the Ansar when Abu Musa came to us as if he were terrified. He said: "Umar ordered me to come to him, so I came to him and asked for permission three times, but I was not given permission, so I returned. And the Messenger of Allah (ﷺ) had said: 'Whoever asks for permission three times and is not given permission, let him return.'" Umar said: "You shall bring proof for what you say, or else I will punish you." Abu Sa’id said: Abu Musa came to us terrified—or he said: alarmed—and said: "I ask you to bear witness." Ubayy ibn Ka’b said: "No one shall stand with you except the youngest of the people." Abu Sa’id said: I was the youngest of them, so I stood with him and testified that the Messenger of Allah (ﷺ) said: "Whoever asks for permission three times and is not given permission, let him return."