Musannaf Ibn Abi Shayba 8:2946

Abu Bakr narrated to us, he said: Yahya bin Sa'id narrated to us, from Ibn Jurayj, from Muhammad bin 'Abbad, from 'Abdullah bin 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.