Reference: Sharh 'Umdatil-Fiqh - Tape no.8 q58
Question: If I entered the masjid after the adhaan of the fajr prayer [has been called], should I pray the two rak'ahs of tahiyyatul-masjid, and then pray the two [Sunnah] rak'ahs of Fajr?
Response: [Praying] the two [Sunnah] rak'ahs of Fajr suffice the tahiyyatul-masjid; [so] pray the Sunnah of the Fajr, as that will be sufficient.
