The Book of Prayers (Kitab Al-Salat)
Muslim :: Book 4 : Hadith 960
Al-Bara' reported: They (the Companions) said prayer with the Messenger of Allah (may peace be upon him), and they bowed when he (the Holy Prophet) bowed. and when he raised his head after bowing, he pronounced:" Allah listened to him who praised Him," and we kept standing till we saw him placing his face on the ground and then we followed him.