Musnad Ahmad 25:866

Husayn ibn Muhammad and Abu Ahmad al-Zubayri narrated to us, they said: Sharik and Hajjaj narrated to us, they said: Sharik 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]: with 'Glorify the name of your Lord, the Most High,' 'Say, O disbelievers,' and 'Say, He is Allah, [who is] One.'"