What to say immediately following Witr prayer – Hadith

Subhaanal-Malikil-Qudoosi [Recite three times in Arabic, and raise and extend the voice on the third time and say…]

Rabbil-malaa`ikati warroohi

Subhaanal-Malikil-Qudoosi means free from any imperfections is the King, the Holy.
Rabbil-malaa`ikati warroohi means Lord of the angels and the Spirit.

An-Nasaai 3/244, Ad-Daraqutni and others. The final addition is from Ad-Daraqutni’s version 2/31 and its chain of narration is authentic.

Source: Fortress of the Muslim, p. 109