Sahih al-Bukhari Book 10, Hadith 17
The Prophet (ﷺ) used to offer two light rak`at between the Adhan and the Iqama of the Fajr prayer.
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.
Allah's Messenger (ﷺ) said, "Bilal pronounces the Adhan at night, so keep on eating and drinking (Suhur) till Ibn Um Maktum pronounces the Adhan."