Dua Remembrance immediately after salam of the witr prayer

سُبْحَانَ الْمَلِكِ الْقُدُّوسِ⁽¹⁾ ، ثلاثَ مرَّاتٍ والثَّالِثَةُ يَجْهَرُ بها ويَمُدُّ بها صَوتَهُ يقولُ: [رَبِّ الْمَلَائِكَةِ وَالرُّوحِ] ⁽²⁾.

The Messenger of Allah (Peace Be Upon Him) would recite (the following chapters) during the witr prayer: ( Sabbih isma rabbika al-aAAla ….) [Al-aAAla] ( Qul ya ayyuha alkafiroon …..) [Al-kafiroon] ( Qul huwa Allahu ahad …..) [Al-ikhlas] …after giving salam he would supplicate three times: Subhanal-malikil-quddoos.(three times).
‘How perfect The King, The Holy One is.’ (three times) …on the third time he would raise his voice, elongate it and add: Rabbil-mala-ikati warrooh. ‘Lord of the angles and the Rooh (i.e. Jibra-eel).