Musnad Ahmad 38:10

'Abd al-Razzaq narrated to us, he said: Sufyan informed us, from Zubayd, from Dharr ibn 'Abd Allah al-Murhabi, from Sa'id ibn 'Abd al-Rahman ibn Abza, from his father, who said: "The Prophet, may Allah bless him and grant him peace, 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 intended to finish the Witr, he would say: 'Subhan al-Malik al-Quddus' (Glory be to the Sovereign, the Holy) three times, and he would raise his voice in the third time."