«كان إذا ركع قال: سبحان ربي العظيم وبحمده - ثلاثا - وإذا سجد قال: سبحان ربي الأعلى وبحمده -» ثلاثا" "
It was the practice of the Prophet (peace and blessings be upon him):
"When he (peace and blessings be upon him) bowed, he would say three times: Subhana Rabbiyal-'Azimi wa bi-hamdih (Glory be to my Lord the Magnificent, and praise be to Him), and when he prostrated, he would say three times: Subhana Rabbiyal-A'la wa bi-hamdih (Glory be to my Lord the Most High, and praise be to Him)."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4734
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[د] عن عقبة بن عامر. صفة الصلاة ١١٣.