Musnad Ahmad 38:4

Bahz narrated to us, Hammam narrated to us, Qatadah informed us, from 'Uzrah, from Sa'id ibn 'Abd al-Rahman ibn Abza, from his father, from the Prophet (peace and blessings of Allah be upon him): "He used to recite in the Witr: '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), extending it three times."