Sahih al-Bukhari Book 10, Hadith 16
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.
My father said that Allah s Apostle said, "Bilal pronounces 'Adhan at night, so keep on eating and drinking (Suhur) till Ibn Um Maktum pronounces Adhan." Salim added, "He was a blind man who would not pronounce the Adhan unless he was told that the day had dawned."
The Prophet (ﷺ) used to offer two light rak`at between the Adhan and the Iqama of the Fajr prayer.