It is narrated from Sayyiduna Abdullah (may Allah be pleased with him): "When the imam says 'Sami'allahu liman hamidah' (Allah has heard the one who praised Him), those following him should say 'Rabbana lakal-hamd' (Our Lord, all praise belongs to You)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2620