Musannaf Ibn Abi Shayba 35:421

Shababa narrated to us, from Yunus, from Abu Ishaq, from Sa'id ibn Jubayr, from Ibn Abbas, that the Prophet (peace and blessings of Allah be upon him) used to perform Witr with three [rak'ahs]. He would recite in them: "Glorify the name of your Lord, the Most High," "Say, 'O disbelievers,'" and "Say, 'He is Allah, [who is] One.'"