Musnad Ahmad 5:298

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Abu al-Tayyah, a man from 'Anazah narrated to me, from a man from Banu Asad, who said: 'Ali went out when the caller for the morning prayer had called, and he said: "The Messenger of Allah (ﷺ) commanded us to pray Witr, so establish it for him at this hour." Then he said: "Perform the Iqamah, O son of al-Nawwahah."