Musannaf Ibn Abi Shayba 3:4523

Abu Bakr narrated to us, he said: Hushaym narrated to us, he said: Mansur informed us, from al-Harith al-'Ukli, from Ibrahim, from al-Aswad ibn Yazid, that Ibn 'Umar used to perform qunut in the witr prayer before the bowing (ruku').