Sahih Bukhari Volume 1, Book 12, Hadith Number 775

Narated 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.