Musnad Ahmad 5:128

Abu Nuh—meaning Qarrad—narrated to us, Shu'bah informed us, from Abu al-Tayyah, who said: I heard Abdullah ibn Abi al-Hudhayl narrating from a man from Banu Asad, who said: Ali ibn Abi Talib came out to us, and they asked him about the Witr prayer. He said: "The Messenger of Allah (peace and blessings of Allah be upon him) commanded us to perform the Witr prayer at this hour. Call out, O Ibn al-Nabbah, or give the Adhan, or perform the Iqamah."