Musnad Ahmad 38:11

Waki’ narrated to us, Sufyan narrated to us, from Zubayd, from Dharr al-Hamadani, from Sa’id ibn ‘Abd al-Rahman ibn Abza al-Khuza’i, from his father: "The Prophet, 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 sat at the end of his prayer, he would say: 'Subhan al-Malik al-Quddus' (Exalted is the Sovereign, the Pure) three times, extending his voice in the last one."