Narrated Hafsa: When the Mu'adh-dhin pronounced the Adhan for Fajr prayer and the dawn became evident the Prophet ordered a two rak`at light prayer (Sunna) before the Iqama of the compulsory (congregational) prayer.
Hadith ReferenceSahih Bukhari / The Book of Adhan / Hadith: 618
Hadith Gradingمحدثین:أحاديث صحيح البخاريّ كلّها صحيحة