Reference : Hisn al-Muslim 33

سُبْـحانَ رَبِّـيَ الْعَظـيم (ثلاث مرات)
Subḥāna Rabbiyal-`Aẓīm.[three times]

Glory to my Lord the Exalted (three times in Arabic)

Reference:
Abu Dawud, Ibn Majah, An-Nasa'i, At-Tirmidhi, and Ahmad. See Al-Albani's 
Sahih At-Tirmidhi 1/83.

Chapter: While bowing in prayer