Sayyiduna Ibn Umar (may Allah be pleased with them both) said: One day the Messenger of Allah (peace and blessings be upon him) led us in prayer. When he (peace and blessings be upon him) raised his head from ruku', he said: "Sami'a Allahu liman hamidah" (Allah hears the one who praises Him). A man behind him (peace and blessings be upon him) said: 'Rabbana wa laka al-hamd, kathiran tayyiban mubarakan fih' (Our Lord, and to You belongs praise, abundant, pure and blessed). When the Prophet (peace and blessings be upon him) finished the prayer, he said three times: "Who spoke just now?" The man said: I did, O Messenger of Allah! He (peace and blessings be upon him) said: "By the One in whose Hand is my soul, I saw more than thirty angels racing one another over this phrase, each trying to be first to write it down."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13543