Sahih Bukhari
Call to Prayers (Adhaan)
Volume: 001 / Book Number: 011 / Hadith Number: 592
Narrated Hafsa:
When the Muadh-dhin pronounced the Adhan for Fajr prayer and the dawn became evident the Prophet ordered a two Rakat light prayer (Sunna) before the Iqama of the compulsory (congregational) prayer.