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.
The two-rak'at light prayer should be performed after hearing the Mu'adh-dhin's call for Fajr but before saying the Iqama for congregation. It should be done with humility and sincerity in order to reap its full benefits spiritually. Furthermore, it is important to remember that this sunnah should not replace or take away from any other obligatory prayers or acts of worship that are required by Islam such as salah or fasting during Ramadan etc.