Musnad Ahmad 38:15

Yahya ibn Sa'id narrated to us, from Shu'bah, Qatadah narrated to us, from Zurarah, from 'Abd al-Rahman ibn Abza: "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' (Surah Al-A'la)."