It was narrated from Sa'eed bin Abdur-Rahman bin Abza that his father said: "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 after he had said the salam, he would say: Subhanal-Malikil-Quddus (Glory be to the Sovereign, the Most Holy) three times, elongating the words the third time, then raising it."
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا حَرَمِيُّ بْنُ يُونُسَ بْنِ مُحَمَّدٍ، قَالَ حَدَّثَنَا أَبِي قَالَ، حَدَّثَنَا جَرِيرٌ، قَالَ سَمِعْتُ زُبَيْدًا، يُحَدِّثُ عَنْ ذَرٍّ، عَنْ سَعِيدِ بْنِ عَبْدِ الرَّحْمَنِ بْنِ أَبْزَى، عَنْ أَبِيهِ، قَالَ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يُوتِرُ بِـ { سَبِّحِ اسْمَ رَبِّكَ الأَعْلَى } وَ { قُلْ يَا أَيُّهَا الْكَافِرُونَ } وَ { قُلْ هُوَ اللَّهُ أَحَدٌ } وَإِذَا سَلَّمَ قَالَ " سُبْحَانَ الْمَلِكِ الْقُدُّوسِ " . ثَلاَثَ مَرَّاتٍ يَمُدُّ صَوْتَهُ فِي الثَّالِثَةِ ثُمَّ يَرْفَعُ .
sunnah.com