Musnad Ahmad 38:5

Abu Dawud narrated to us, Shu'bah narrated to us, Qatadah informed us, he said: I heard Zurarah narrating from 'Abd al-Rahman ibn Abza: "That the Messenger of Allah (peace and blessings of Allah be upon him) used to perform Witr with '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). And when he would say the Taslim, he would say: 'Subhan al-Malik al-Quddus' (Glory be to the Sovereign, the Holy), saying it three times."