Musnad Ahmad 25:1046

Hajjaj narrated to us, Sharik informed us, from Abu Ishaq, from Sa'id ibn Jubayr, from Ibn Abbas, who said: "The Prophet of Allah, may Allah bless him and grant him peace, used to perform Witr with three [rak'ahs]: with 'Glorify the name of your Lord, the Most High,' and 'Say, O disbelievers,' and 'Say, He is Allah, [who is] One.'"