Sahih Bukhari | Volume 1 | Hadith Number 775

Narrated by Al-Bara' bin 'Azib
(And he was not a liar) We used to pray behind the Prophet and when he said, "Sami' a-l- lahu Liman hamida", none of us would bend his back (to go for prostration) till the Prophet had placed his, forehead on the ground.