Musnad Ahmad 38:7

'Affan narrated to us, Shu'bah narrated to us, he said: Zubayd and Salamah informed me that they heard Dharr, from Ibn 'Abd al-Rahman ibn Abza, from his father, that the Prophet (ﷺ) 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" (Exalted is the Sovereign, the Pure), three times, raising his voice in the last one.