Riyad as-Salihin, Book 16, Hadith 1425 (In-Book)

'Aishah (May Allah be pleased with her) reported: The Prophet used to recite frequently in his bowing and prostration: "Subhanak- Allahumma, Rabbana wa bihamdika. Allahum-maghfir li (O Allah! You are free from imperfection and I begin with praising You. Forgive my sins)."[Al-Bukhari and Muslim].

وعن عائشة رضي الله عنه قالت‏:‏ كان النبي صلى الله عليه وسلم يكثر أن يقول في ركوعه وسجوده‏:‏ سبحانك اللهم ربنا وبحمدك، اللهم اغفر لي‏"‏ ‏(‏‏(‏متفق عليه‏)‏‏)‏‏.‏

sunnah.com