(12) باب الأَذَانِ بَعْدَ الْفَجْرِ
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.
Reference: Sahih al-Bukhari 618
In-book reference: Book 10, Hadith 16
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 1, Book 11, Hadith 592