Allah's Messenger said: "When the Imam says: (Sami Allahu liman hamidah) 'Allah listens to those who praise Him. Then (all of you) say: (Rabbana wa lakal-hamd) 'O our Lord! And to You is the praise for whoever's saying concurs with the saying of the angels, then his past sins will be forgiven."
Jami` at-Tirmidhi Book 2, Hadith 119
← Previous Hadith "When Allah's Messenger would raise his head from bowing he would say: (Sami Allahi liman hamidal. Rabba na lakal-hamd, mil'as-samawati wa mil'al-ardi wa mil'a ma bainahuma wa mil'a ma shi'ia min shay'in ba'd) 'Allah listens to those who praise Him. O our Lord! And to You is the praise that fills the heavens and fills the earth, and fills what is between them, and fills whatever there is beyond that You will.' He said: There are narrations on this topic from Ibn Umar, Ibn Abbas, Ibn Abi Awfa, Abu Juhaifa, and Abu Sa'eed.

Jami` at-Tirmidhi Book 2, Hadith 118 Next Hadith → "I saw Allah's Messenger when he prostrated placing his knees (on the ground) before his hands, and when he got up, he raised his hands before his knees."

Jami` at-Tirmidhi Book 2, Hadith 120