Sunan an-Nasa'i, Hadith 1048 ⓘ
It was narrated that Aishah said: "The Messenger of Allah used to say when bowing: 'Subbuhun Quddusun Rabbul-mala'ikati war-ruh (Perfect, Most Holy, is the Lord of the angels and the spirit).'"
أَخْبَرَنَا مُحَمَّدُ بْنُ عَبْدِ الأَعْلَى، قَالَ حَدَّثَنَا خَالِدٌ، قَالَ حَدَّثَنَا شُعْبَةُ، قَالَ أَنْبَأَنِي قَتَادَةُ، عَنْ مُطَرِّفٍ، عَنْ عَائِشَةَ، قَالَتْ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَقُولُ فِي رُكُوعِهِ " سُبُّوحٌ قُدُّوسٌ رَبُّ الْمَلاَئِكَةِ وَالرُّوحِ " .
also referenced under: Book 12, Hadith 20 (In-Book) & Volume 2, Book 12, Hadith 1049 (DarusSalam)