Musannaf Ibn Abi Shayba 21:593

Muhammad ibn Abi Ubaydah narrated to us, he said: My father narrated to us, from al-A’mash, from Talhah, from Zirr, from Sa’id ibn Abd al-Rahman ibn Abza, from his father, from Ubayy ibn Ka’b, that the Prophet, peace and blessings of Allah be upon him, used to say at the end of his prayer: "Subhan al-Malik al-Quddus" (Exalted is the Sovereign, the Holy), three times.