Musannaf Ibn Abi Shayba 3:4558

Muhammad ibn Bishr narrated to us, he said: Sa’id narrated to us, from Qatadah, from al-Hasan, that Ubayy led the people in prayer during the caliphate of Umar. He prayed with them for the first half of Ramadan and did not perform qunut. When the half had passed, he performed qunut after the bowing (ruku’). When the last ten days entered, he left them and withdrew from them, and Mu’adh al-Qari led them in prayer for the last ten days during the caliphate of Umar.