Hadithcore
Mishkat al-Masabih, 1274, 1275

Ubayy b. Ka'b said that when God’s Messenger gave the salutation in the <i>witr</i> he said, “Glory be to the King most holy.” Abu Dawud and Nasa’i transmitted it, Nasa’i adding that he said it three times, prolonging the words. In a version by Nasa’i from ‘Abd ar-Rahman b. Abza from his father, he said that he used to say three times when he uttered that salutation, “Glory be to the King most holy”, raising his voice when saying it the third time.

وَعَنْ أُبَيِّ بْنِ كَعْبٍ قَالَ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِذَا سَلَّمَ فِي الْوتر قَالَ: «سُبْحَانَكَ الْمَلِكِ الْقُدُّوسِ» رَوَاهُ أَبُو دَاوُدَ وَالنَّسَائِيُّ وَزَادَ: ثَلَاث مَرَّات يُطِيل فِي آخِرهنَّ وَفِي رِوَايَةٍ لِلنَّسَائِيِّ عَنْ عَبْدِ الرَّحْمَنِ بْنِ أَبْزَى عَنْ أَبِيهِ قَالَ: كَانَ يَقُولُ إِذَا سَلَّمَ: «سُبْحَانَ الْمَلِكِ الْقُدُّوسِ» ثَلَاثًا وَيَرْفَعُ صَوْتَهُ بالثالثة

References1 variant
In-Book Reference
Book 4, Hadith 686