Shaykh al-Islām was asked:

If someone arrives when the call to commence prayer has already been made, which is better: to join the obligatory prayer, or to offer the Sunnah prayer first and then join the imam, even if only during the tashahhud? Are the two rak'ahs of dawn a Sunnah prayer associated with the obligatory morning prayer?

He replied:

It is authentically reported that the Prophet (may Allah bless him and grant him peace) said:

“When the call to commence prayer is made, no prayer is to be offered except the obligatory prayer.”

Another narration states:

“No prayer is to be offered except the one for which the call has been made.”

Once the call to commence prayer has been made, one should not occupy oneself with either the prayer of greeting the mosque or the Sunnah prayer of dawn. The scholars agree that one should not occupy oneself with the prayer of greeting the mosque instead of joining the obligatory prayer. They differ, however, concerning the Sunnah prayer of dawn.

The correct view is that, upon hearing the call to commence prayer, one should not offer the Sunnah prayer, whether at home or elsewhere. Rather, one may make it up after the obligatory prayer if one wishes.

The Sunnah is to offer two rak'ahs as a Sunnah prayer after dawn breaks, while the obligatory prayer consists of two rak'ahs. Between the break of dawn and the obligatory prayer, there is no Sunnah prayer other than these two rak'ahs. The obligatory prayer is called both the dawn prayer and the morning prayer. Likewise, the Sunnah prayer is called the Sunnah of dawn, the Sunnah of the morning, the two rak'ahs of dawn, and other similar names. And Allah knows best.