Musannaf Ibn Abi Shayba 3:4496

Waki’ narrated to us, from Sufyan, from Zubayd, from Zirr, from Sa’id ibn ‘Abd al-Rahman ibn Abza, from his father, that the Prophet (may Allah bless him and grant him peace) 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.'" And he would say at the end of his prayer, when he sat, "Subhan al-Malik al-Quddus" (Glory be to the Sovereign, the Holy) three times, extending his voice with it in the last one.