Musannaf Ibn Abi Shayba 3:4564

Sahl ibn Yusuf narrated to us, from ‘Amr, from al-Hasan, that when ‘Umar ordered Ubayy to lead the people in prayer during Ramadan, and ordered him to perform the Qunut with them in the remaining half, starting from the night of the sixteenth. He said: And al-Hasan used to say: If he is an Imam, he performs the Qunut in the [second] half, and if he is not an Imam, he performs the Qunut the entire month.