Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 958
Al-Bara' (b. 'Azib), and he was no liar (but a truthful Companion of the Holy Prophet), reported: They used to say prayer behind the Messenger of Allah (may peace be upon him). I never saw anyone bending his back at the time when he (the Holy Prophet) raised his head, till the Messenger of Allah (may peace be upon him) placed his forehead on the ground. They then fell in prostration after him.