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 Referenceصحيح البخاري / كتاب الأذان / 618
Hadith Gradingمحدثین:أحاديث صحيح البخاريّ كلّها صحيحة
Narrated `Abdullah bin `Umar: Allah's Apostle said, "Bilal pronounces the Adhan at night, so keep on eating and drinking (Suhur) till Ibn Um Maktum pronounces the Adhan."
Hadith Referenceصحيح البخاري / كتاب الأذان / 620
Hadith Gradingمحدثین:أحاديث صحيح البخاريّ كلّها صحيحة