Musannaf Ibn Abi Shayba 3:4502

Waki’ narrated to us, from Isra’il, from Abu Ishaq, from Sa’id ibn Jubayr, from Ibn Abbas, that he used to perform Witr with three [rak'ahs]: "Glorify the name of your Lord, the Most High" (Surah Al-A'la), "Say, 'O disbelievers'" (Surah Al-Kafirun), and "Say, 'He is Allah, [who is] One'" (Surah Al-Ikhlas).