Musnad Ahmad 30:2795

I read to Abd al-Rahman: Malik, and Ishaq narrated to us, he said: Malik informed us, from Sumayy, the freed slave of Abu Bakr—meaning Ibn Abd al-Rahman—from Abu Salih al-Samman, from Abu Hurairah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "When the Imam says: 'Sami' Allahu liman hamidah' (Allah hears those who praise Him), then say: 'Allahumma Rabbana laka al-hamd' (O Allah, our Lord, to You is the praise). For whoever’s saying coincides with the saying of the angels, his past sins will be forgiven."