Sahih Al Bukhari Hadith Number 774

Narrated Ibn `Abbas: The Prophet (peace be upon him) recited aloud in the prayers in which he was ordered to do so and quietly in the prayers in which he was ordered to do so. “And your Lord is not forgetful.” “Verily there was a good example for you in the ways of the Prophet.”

10  Call to Prayers (Adhaan)
Hadith number 774
Volume no 1
Chapter: 105 (To recite aloud in the Fajr (early morning) prayer.)