Musnad Ahmad 38:8

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Qatadah, from 'Uzrah, from Sa'id ibn 'Abd al-Rahman ibn Abza, from his father: "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 he would say when he finished with the Taslim: 'Subhan al-Malik al-Quddus' (Exalted is the Sovereign, the Holy) three times."