Reference : Hisn al-Muslim 41

سُبْـحانَ رَبِّـيَ الأَعْلـى. (ثلاث مرات)
Subḥāna Rabbiya ‘l-a`lā.

Glory is to my Lord, the Most High. (This is said three times in Arabic.)

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

Chapter: While prostrating