«كان إذا كان راكعا أو ساجدا قال: سبحانك وبحمدك أستغفرك وأتوب إليك»
It was the practice of the Prophet (peace and blessings be upon him):
"When he (peace and blessings be upon him) was in bowing (ruku') or prostration, he would say: O Allah, You are free of all imperfection and Yours alone is the praise; I seek Your forgiveness and I turn to You in repentance."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4771