It is narrated from Sayyiduna Rifa'ah bin Rafi' al-Zuraqi (may Allah be pleased with him): 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'allahu liman hamidah', a man behind the Messenger of Allah (peace and blessings be upon him) said: 'Rabbana wa lakal-hamd, hamdan kathiran tayyiban mubarakan fih' (Our Lord, and to You belongs all praise, abundant, pure and blessed praise). When he (peace and blessings be upon him) said the salam, he asked: "Who was the one who said these words?" The man said: O Messenger of Allah, it was I. He (peace and blessings be upon him) said: "I saw more than thirty angels racing one another (to record its reward), each wanting to be the first to write it down."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2610