Authentic Supplication Number 052.

It is reported by way of 'Ibn `Abdirrahman bin 'Abzee by way of his father about the Prophet  that he used to read, in the Witr [prayer]: Sabbih 'isma rabbikal 'a`la, Qul ya 'ayyuhal kafiroona, and Qul huwa 'allahu 'ahadun (73), then... when he would have said salam, he would say: Subhana 'al-maliki 'al-quddoosi, Subhana 'al-maliki 'al-quddoosi, Subhana 'al-maliki 'al-quddoosi (74), and he raised his voice with that one (75) [meaning the voice is raised with the last one].

Footnotes.
(73) These are chapters 87, 109, 112 of the Qur'an.
(74) Glorified is The Holy Sovereign.
(75) In a report by way of Sa`eed bin `Abdirrahman bin 'Abzee "he would prolong it three times", and in another report (V3. p407) in the Musnad of Imam 'Ahmad he would extend his voice with the last one.

Reporters.
'Al-`Adawee said that it was reported by Imam 'Ahmad, page 406 of volume 3 of Musnad, and he said that it was sound. He also said that a report by way of Sa`eed bin `Abdirrahman bin 'Abzee said: "he would prolong it three times", and in another report found on page 407 of volume 3 of the Musnad of Imam 'Ahmad he would extend his voice with the last one.