Musnad Ahmad 5:297

Hashim ibn al-Qasim narrated to us, Shu'bah narrated to us, from Abu al-Tayyah, who said: I heard a man from 'Anazah narrating from a man from Banu Asad, who said: Ali came out to us and said: "The Prophet, may Allah bless him and grant him peace, commanded the Witr; his Witr is established at this hour." "O Ibn al-Nabbah, give the Adhan, or give the Iqamah."