Sahih Bukhari
Call to Prayers (Adhaan)
Volume: 001 / Book Number: 011 / Hadith Number: 658
Narrated Al-Bara:
(and he was not a liar) When Allah's Apostle said, "Sami a-l-lahu Liman hamida " none of us bent his back (for prostrations) till the Prophet prostrated and then we would prostrate after him.