Is it obligatory to pray Fajr and Subh together after hearing the Adhan for Subh, or is it obligatory to pray Fajr before the Adhan and Subh after it?

Fajr prayer is the Subh prayer, and some people refer to the two Rak'ahs of the Sunnah prayer that precede the Fajr prayer by this name. These two Rak'ahs are an emphasized Sunnah and are not valid unless performed after the time for the obligatory prayer has begun.