Musannaf Ibn Abi Shayba 3:5326

Shababa narrated to us, he said: Layth ibn Sa’d narrated to us, from Ibn Shihab, from ‘Urwah, from ‘Abd al-Rahman ibn al-Qari, who said: ‘Umar ibn al-Khattab went out in the month of Ramadan, and the people were praying in separate groups. He said: "If we were to gather these people behind one reciter, it would be better." So he gathered them behind Ubayy ibn Ka’b.