Musnad Ahmad 38:3

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Salamah ibn Kuhayl and Zubayd al-Yami, from Dharr, from Ibn 'Abd al-Rahman ibn Abza, from his father, from the Prophet, may Allah bless him and grant him peace: "That he used to recite in the Witr [prayer]: 'Glorify the name of your Lord, the Most High' (Surah Al-A'la), 'Say, O disbelievers' (Surah Al-Kafirun), and 'Say, He is Allah, [who is] One' (Surah Al-Ikhlas). And when he would say the taslim, he would say: 'Exalted is the Sovereign, the Holy; Exalted is the Sovereign, the Holy; Exalted is the Sovereign, the Holy,' and he would raise his voice with it."