What to say immediately following the Witr prayer

 "سُبْحَانَ الْمَلِكِ الْقُدُّوْسِ"

119: Subhaanal-Malikil-Quddoosi.

Glory is to the King, the Holy. [Recite three times in Arabic, and raise and extend the voice on the third time and say . . .] 

"رَبِّ الْمَلَائِكَةِ وَالرُّوحِ".

Rabbil-malaa'ikati warroohi. 

Lord of the angels and the Spirit.

Reference: 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.