The Messenger of Allah (ﷺ) used to say while bowing down and prostration:

سُبُّوحٌ، قُدُّوسٌ، رَبُّ الْمَلَائِكَةِ وَالرُّوحِ

Subboohun, Quddoosun, Rabbul-malaa'ikati warrooh

Glory (to You), Most Holy (are You), Lord of the angels and the Spirit.

Reference: Sahih (Albani). Abu Dawud: 872