Sunan an-Nasa'i, Hadith 1734 ⓘ
Mansur reported from Salamah bin Kuhail, from Sa'eed bin Abdur-Rahman bin Abza, from his father, who said: "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 مُحَمَّدُ بْنُ قُدَامَةَ، عَنْ جَرِيرٍ، عَنْ مَنْصُورٍ، عَنْ سَلَمَةَ بْنِ كُهَيْلٍ، 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 } وَكَانَ إِذَا سَلَّمَ وَفَرَغَ قَالَ " سُبْحَانَ الْمَلِكِ الْقُدُّوسِ " . ثَلاَثًا طَوَّلَ فِي الثَّالِثَةِ . وَرَوَاهُ عَبْدُ الْمَلِكِ بْنُ أَبِي سُلَيْمَانَ عَنْ زُبَيْدٍ وَلَمْ يَذْكُرْ ذَرًّا .
also referenced under: Book 20, Hadith 137 (In-Book) & Volume 2, Book 20, Hadith 1735 (DarusSalam)