After giving salaam from the Witr Prayer, Prophet saw would supplicate:

سُبْحَانَ المَلِكِ القُدُّوسِ

Glory is to the King, the Holy. [1]

Subhaanal-Malikil-Quddoosi.

...three times,

and then raise and extend the voice on the third time and say:

رَبِّ الْمَلاَئِكَةِ وَالرُّوحِ

Lord of the angels and the Spirit. [2]

Rabbil-malaa'ikati warroohi.

[1] An-Nasa'i 3/244, Ad-Daraqutni and others. The final addition is from Ad-Daraqutni's version 2/ 31 and its chain of narration is authentic. See the checking of Zadul-Ma'ad by Shu'aib Al-Arna'ut and 'Abdul-Qadir Al-Arna'ut 1/337. [2] Abu Dawud 2/87. See also Al-Albani, Sahih Ibn Mdjah 2/335