Musannaf Ibn Abi Shayba 35:420

Muhammad ibn Abi Ubaydah narrated to us, my father narrated to us, from al-A’mash, from Talhah, from Dharr, 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 perform Witr with "Glorify the name of your Lord, the Most High," and "Say, 'O disbelievers,'" and "Say, 'He is Allah, [who is] One.'"