It is narrated from Sa'id bin Abi Sa'id that he heard Abu Hurayrah (may Allah be pleased with him), while he was leading the congregation, say (on rising from bowing) 'Sami'allahu liman hamidah' (Allah has heard the one who praised Him), 'Allahumma rabbana lakal-hamd' (O Allah, our Lord, to You belongs praise), then he said 'Allahu akbar' (Allah is the Greatest). He (may Allah be pleased with him) would say these words in a loud voice, and we would say them too, following him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2615