Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: "When the imam says 'Sami' Allahu liman hamidah', those behind him should say: 'Rabbana laka al-hamd (Our Lord, to You belongs all praise).'"
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9291