Musannaf Ibn Abi Shayba 3:5343

Abu Bakr narrated to us, he said: Yahya ibn Sa'id narrated to us, from Ibn Jurayj, from Muhammad ibn 'Abbad, from 'Abdullah ibn al-Sa'ib, he said: I was leading the people in prayer during Ramadan. While I was praying, I heard the takbir of 'Umar at the door of the mosque. He had arrived to perform 'Umrah, so he entered and prayed behind me.