When he enters the
    mosque, should he pray two Rak’ahs of Tahiyyat al-Masjid?
Question

                When the imaam or a member of the
      congregation enters the mosque, should they pray Tahiyyat al-Masjid if they have already
      prayed Sunnat al-Fajr at home and then come to the mosque to pray Fajr?
Praise be to Allah.If the situation is as you describe, then when they
    enter the mosque it is prescribed for them to pray Tahiyyat al-Masjid before sitting down.
    The imaam may order that the Iqaamah be given for Fajr prayer, after which there is no
    need for Tahiyyat al-Masjid, because praying the fard (obligatory) prayer is sufficient
    and means that there is no longer any need to do Tahiyyat al-Masjid. The member of the
    congregation may also forego Tahiyyat al-Masjid if he enters the mosque when the Iqaamah
    is being given.
And Allaah is the source of strength. May Allaah bless our Prophet
    Muhammad and his family and companions.