Musannaf Ibn Abi Shayba 3:4481

Abu Khalid al-Ahmar narrated to us, from Hajjaj, from ‘Amr ibn Shu‘ayb, from his father, from his grandfather, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Indeed, Allah has added for you a prayer in addition to your prayers, and it is the Witr."