Musnad Ahmad 922:7

Abdullah narrated to us, Abu Bakr ibn Abi Shaybah narrated to me, Muhammad ibn Abi Ubaydah narrated to us, from my father, from al-A’mash, from Talhah al-Yami, from Dharr, from Ibn Abd al-Rahman ibn Abza, from his father, from Ubayy ibn Ka’b, who said: The Messenger of Allah (peace and blessings of Allah be upon him) used to recite in the Witr prayer: "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.