Musnad Abu Ya'la al-Mawsili 110:229

Zuhayr narrated to us, Shababah ibn Sawwar narrated to us, Yunus narrated to us, from Abu Ishaq, from Sa'id ibn Jubayr, from Ibn Abbas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) used to perform Witr with three [rak'ahs]: "Glorify the name of your Lord, the Most High," "Say, 'O disbelievers,'" and "Say, 'He is Allah, [who is] One.'"