Is it permissible to sit upon entering the mosque when the prayer is about to begin, or is it better to remain standing?

If you enter the mosque before the obligatory prayer is commenced, pray "Tahiyyat al-Masjid" (two rak'ahs upon entering the mosque) and do not sit down, as the Prophet, peace and blessings be upon him, said: "If any of you enters the mosque, let him not sit down until he has prayed two rak'ahs." The prohibition of performing a supererogatory prayer after the commencement of the obligatory prayer is based on the saying of the Prophet, peace and blessings be upon him: "When the prayer has been commenced, there is no prayer except the obligatory one." If the obligatory prayer is commenced while you are praying "Tahiyyat al-Masjid," the preponderant opinion is that you should cut it short and join the Imam.