Musnad Ahmad 1153:1888

Muhammad ibn Salamah narrated to us, from Khasif, from Abd al-Aziz ibn Jurayj, who said: I asked Aisha, the Mother of the Believers, "With what did the Messenger of Allah, peace and blessings of Allah be upon him, perform the Witr prayer?" She said: "He used to recite in the first rak'ah 'Glorify the name of your Lord, the Most High' (Surah Al-A'la), and in the second 'Say, O disbelievers' (Surah Al-Kafirun), and in the third 'Say, He is Allah, [who is] One' (Surah Al-Ikhlas) and the two refuge-seeking Surahs (Al-Mu'awwidhatayn)."