Is it obligatory to pray in the mosque with the congregation, or to pray before them, taking into consideration the spread of light at the time of prayer?

The reason for mosques in Turkey delaying the Fajr prayer until the sky brightens is that the prevailing jurisprudential school there is the Hanafi school. They hold the view that it is preferable to delay Fajr until isfar, meaning the spread of whiteness across the horizon, contrary to the majority who deem it preferable to pray Fajr at the beginning of its time (taghlees).

However, the Hanafis do not consider it permissible to delay it to a time that would cause doubt about sunrise. The time for Fajr prayer begins with the true dawn (Fajr al-Sadiq) and ends with sunrise. So, if the mosques perform the prayer before sunrise, it is within its prescribed time, even if it is preferable to hasten it. But if it is performed after sunrise, that is not permissible. Prayer is obligatory within its legally specified time, as Allah Almighty says: "Indeed, prayer has been decreed upon the believers a time-bound ordinance."