"Shu'bah narrated to us, from Salamah and Zubaid, from Dharr, from Ibn Abdur-Rahman bin Abza from his father that the Messenger of Allah ﷺ used to recite in Witr: "Glorify the Name of your Lord, the Most High;" and "Say: O you disbelievers!;' and 'Say: He is Allah, (the) One.' And when he said the taslim, he would say: Subhanal-Malikil-Quddus (Glory be to the Sovereign, the Most Holy) three times, raising his voice the third time." (Using translation from Nasāʾī 1732)
أَنَّ رَسُولَ اللهِ ﷺ كَانَ يُوتِرُ بِسَبِّحِ اسْمَ رَبِّكَ الْأَعْلَى وَقُلْ يَا أَيُّهَا الْكَافِرُونَ وَقُلْ هُوَ اللهُ أَحَدٌ وَكَانَ يَقُولُ إِذَا سَلَّمَ «سُبْحَانَ الْمَلِكِ الْقُدُّوسِ» ثَلَاثًا يَرْفَعُ صَوْتَهُ بِالثَّالِثَةِ