Sahih Muslim Book 4, Hadith Number 960.

Chapter : Moderation in the articles of prayers and their shortening and perfection.

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.