Musannaf Ibn Abi Shayba 3:4664

Abu Bakr narrated to us, he said: Yahya bin Sa’id narrated to us, from Ja’far bin Maymun, from Abu ‘Uthman, who said: ‘Umar used to perform Qunut with us after the bowing (ruku’), and he would raise his hands until his armpits became visible. And his voice would be heard from behind the mosque.