Muwatta Imam Malik (Riwayat Ibn al-Qasim)

It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that when the Messenger of Allah (peace and blessings be upon him) began the prayer, he would raise his hands up to his shoulders, and when he bowed (for ruku‘), and when he raised his head from ruku‘, he would raise his hands in the same way and say: “«سَمِعَ اللَّهُ لِمَنْ حَمِدَهُ » Allah has heard the one who praises Him «رَبَّنَا وَلَكَ الْحَمْدُ» O our Lord! And all praise is for You.” And the Prophet (peace and blessings be upon him) would not raise his hands during prostrations (sujood).

It is narrated from Sayyiduna Rifa’ah bin Rafi’ al-Zuraqi (may Allah be pleased with him) that one day we were praying behind the Messenger of Allah (peace and blessings be upon him). When the Messenger of Allah (peace and blessings be upon him) raised his head from bowing (ruku’) and said: «سَمِعَ اللهُ لِمَنْ حَمِدَهُ» “Allah has heard the one who praises Him,” a man praying behind the Messenger of Allah (peace and blessings be upon him) said: «رَبَّنَا وَلَكَ الْحَمْدُ حَمْدًا كَثِيرًا طَيِّبًا مُبَارَكًا فِيهِ» “O our Lord! And to You alone is all praise, abundant, pure, and blessed.” When the Messenger of Allah (peace and blessings be upon him) finished the prayer with salam, he said: “Who spoke just now (in the prayer)?” A man said: “I did, O Messenger of Allah!” So the Messenger of Allah (peace and blessings be upon him) said: “I saw more than thirty (30) angels racing to be the first to write it down.”

And with the same chain of narration, it is narrated from (Sayyiduna Abu Hurairah, may Allah be pleased with him) that the Messenger of Allah, peace and blessings be upon him, said: "When the Imam says «سَمِعَ اللَّهُ لِمَنْ حَمِدَهُ », then all of you say «اللَّهُمَّ رَبَّنَا لَكَ الْحَمْدُ», because whoever's saying coincides with the saying of the angels, his previous sins are forgiven."