Sayyiduna Rifa'ah bin Rafi' (may Allah be pleased with him) said: One day we were praying behind the Messenger of Allah (peace and blessings be upon him). When he (peace and blessings be upon him) raised his head from bowing and said: "Sami'a Allahu liman hamidah," a man behind him said: "Rabbana wa lakal-hamd, hamdan kathiran tayyiban mubarakan fih" (Our Lord, to You belongs all praise, abundant, pure and blessed praise). When the Messenger of Allah (peace and blessings be upon him) finished the prayer, he said: "Who said that just now?" The man said: I did, O Messenger of Allah. The Messenger of Allah (peace and blessings be upon him) said: "I saw more than thirty angels hastening to see which of them would write it down first."
Hadith Referenceالمعجم الكبير للطبراني / باب الراء / 4516