The Prophet (ﷺ) used to recite this dhikr three times at the end of the Witr prayer, and the last time he would say it at length:

سُبْحَانَ الْمَلِكِ الْقُّدُّوْسِ

Subhaanal-Malikil-Quddoos

Glory is to the King, the Holy.

Reference: Sahih (Albani). Abu Dawud No. 1430; An-Nasa'i No. 1699, 1701