It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When one of you bows and says 'Subhana rabbiyal-'azim' (Glory be to my Lord, the Most Great) three times, his bowing is complete, and this is its minimum. When one of you prostrates and says 'Subhana rabbiyal-a'la' (Glory be to my Lord, the Most High) three times, his prostration is complete, and indeed this is its minimum."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2689