Musannaf Ibn Abi Shayba 3:4579

Abu Bakr narrated to us, he said: Hushaym narrated to us, from Mughirah, from Ibrahim, who said: I entered upon al-Aswad one night while he was ill, and he prayed the Witr while a man was supporting him. He said: So he performed the Qunut and prolonged the Qunut until I thought that he had exceeded what he used to do.