Musnad Ahmad 28:463

Yazid ibn Harun narrated to us, Hajjaj ibn Artah informed us, from Amr ibn Shu'ayb, from his father, from his grandfather, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, Allah the Almighty and Majestic has added a prayer for you, and it is the Witr."