as-Sub-booh (The Justly Glorified One)

On the authority of ‘Aa’ishah (may Allaah be pleased with her), the Prophet (may Allaah raise his rank and grant him peace) used to say in his bowings and in his prostrations (during formal prayer):
“Sub-booh, the Holy One, the Lord of the Angels and the Spirit (Jibreel).”
Collected by Muslim (487)
Reference:
Exemplary principles concerning the beautiful names of Allaah.
Shaykh Muhammad Ibn Saalih Al-‘Uthaymeen D. 1421 H
Translated by Moosaa Richardson
Revised 2nd Edition with additional commentary.

‘A’isha reported that the Messenger of Allah (Peace and blessing of Allaah be upon him) used to pronounce while bowing and prostrating himself:

حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا مُحَمَّدُ بْنُ بِشْرٍ الْعَبْدِيُّ، حَدَّثَنَا سَعِيدُ بْنُ أَبِي عَرُوبَةَ، عَنْ قَتَادَةَ، عَنْ مُطَرِّفِ بْنِ عَبْدِ اللَّهِ بْنِ الشِّخِّيرِ، أَنَّ عَائِشَةَ، نَبَّأَتْهُ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَانَ يَقُولُ فِي رُكُوعِهِ وَسُجُودِهِ ‏ “‏ سُبُّوحٌ قُدُّوسٌ رَبُّ الْمَلاَئِكَةِ وَالرُّوحِ‏”‏‏.‏

Reference: www.Sunnah.com
Reference: Sahih Muslim 487 a
In-book reference: Book 4, Hadith 253
USC-MSA web (English) reference: Book 4, Hadith 987
(deprecated numbering scheme)