Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 959
Al-Bara' reported, and he was no liar: When the Messenger of Allah (may peace be upon him) said: Allah listened to him who praised Him, none of us bent his back till he (the Holy Prophet) prostrated; we then, afterwards, went down in prostration.