Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: When the imam says 'Sami'a Allahu liman hamidah' (Allah hears the one who praises Him), those behind him should say 'Rabbana laka al-hamd' (Our Lord, to You belongs all praise).
Imam al-Tabarani recorded this narration in al-Mu'jam al-Kabir, and its narrators have been declared trustworthy.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2742