Sunan an-Nasa'i, Volume 2, Book 20, Hadith 1742 (DarusSalam) ⓘ

Ishawq bin Mansur informed us, he said: "Abu Dawud narrated to us, he said: Shu'ba narrated to us, from Qatadah, from Abdur-Rahman bin Abza, that The Messenger of Allah 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, elongating his words the third time."

أَخْبَرَنَا إِسْحَاقُ بْنُ مَنْصُورٍ، قَالَ حَدَّثَنَا أَبُو دَاوُدَ، قَالَ حَدَّثَنَا شُعْبَةُ، عَنْ قَتَادَةَ، عَنْ زُرَارَةَ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ أَبْزَى، عَنْ رَسُولِ اللَّهِ صلى الله عليه وسلم كَانَ يُوتِرُ بِـ ‏{‏ سَبِّحِ اسْمَ رَبِّكَ الأَعْلَى ‏}‏ وَ ‏{‏ قُلْ يَا أَيُّهَا الْكَافِرُونَ ‏}‏ وَ ‏{‏ قُلْ هُوَ اللَّهُ أَحَدٌ ‏}‏ فَإِذَا فَرَغَ قَالَ ‏"‏ سُبْحَانَ الْمَلِكِ الْقُدُّوسِ ‏"‏ ‏.‏ ثَلاَثًا وَيَمُدُّ فِي الثَّالِثَةِ ‏.‏