Musnad Ahmad 25:917

Ibrahim ibn Abi al-Abbas narrated to us, Sharik narrated to us, from Makhul, from Muslim al-Batin, 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' (Surah Al-A'la), 'Say, O disbelievers' (Surah Al-Kafirun), and 'Say, He is Allah, [who is] One' (Surah Al-Ikhlas)."