Musannaf Ibn Abi Shayba 21:592

Waki’ narrated to us, from Sufyan, from Zubayd, from Zirr, from Sa’id ibn ‘Abd al-Rahman ibn Abza, from his mother, that the Prophet (peace and blessings of Allah be upon him) used to perform Witr, and he would recite at the end of his prayer when he sat: "Subhan Allah al-Malik al-Quddus" (Glory be to Allah, the Sovereign, the Holy) three times, extending his voice with it.