Musannaf Ibn Abi Shayba 3:4529

Hafs narrated to us, from al-Hasan ibn Ubayd Allah, from Ibrahim, from al-Aswad, who said: I nursed him. He performed the Witr prayer. When he finished the recitation, I waited for him to bow, but he did not do so until he performed the Qunut, then he bowed.