Al-Sa'di narrates from his father or his uncle (may Allah be pleased with him) that he said: I watched the Prophet (peace and blessings be upon him) closely while he prayed; in his bowing and prostrations he would remain for as long as it takes to say 'Subhana Allahi wa bihamdih' (Glory be to Allah, and with His praise) three times.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2557