Sunan an-Nasa'i 1048
سنن النسائي#1048SahihIt was narrated that Aishah said:"The Messenger of Allah (ﷺ) used to say when bowing: 'Subbuhun Quddusun Rabbul-mala'ikati war-ruh (Perfect, Most Holy, is the Lord of the angels and the spirit)
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahSahih
Al-AlbaniSahih
Zubair Ali ZaiSahih Muslim
In-book reference: 12:20