Sahih Bukhari
Sahih Bukhari Volume 001, Book 012, Hadith Number 775.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 12. Charecteristics Of Prayer
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.