Musnad Ahmad 788:97

Yahya—who is Ibn Sa’id—narrated to us, from Ibn Jurayj, from ‘Ata’, from ‘Ubayd ibn ‘Umayr, that Abu Musa sought permission to enter upon ‘Umar, may Allah be pleased with him, three times, but he did not grant him permission, so he returned. ‘Umar said, "Did I not hear the voice of ‘Abdullah ibn Qays just now?" They said, "Yes." He said, "Then seek him out." He said, "So they sought him out, and he was summoned." ‘Umar said, "What compelled you to do what you did?" He said, "I sought permission three times and was not granted permission, so I returned. We used to be commanded to do this." ‘Umar said, "You shall bring proof for this, or I will certainly do [something] to you." He said, "So he went to a mosque or a gathering of the Ansar, and they said, 'None shall testify for you except the youngest among us.'" Abu Sa’id al-Khudri stood up and testified for him. ‘Umar, may Allah be pleased with him, said, "This matter of the Messenger of Allah, peace and blessings of Allah be upon him, was hidden from me; trading in the marketplaces distracted me from it."