What is the ruling on the verbal greeting of the mosque without prayer before the Fajr prayer?

It is sunnah (definition) for one who enters the mosque to pray two rak'ahs before sitting, based on the Prophet's (peace be upon him) saying: "When one of you enters the mosque, he should not sit until he has prayed two rak'ahs." If one enters the mosque after the Fajr adhan (definition) and has not prayed the two rak'ahs of Fajr (Raghibat al-Fajr), he should pray them, and they will suffice for the two rak'ahs of greeting the mosque (Tahiyat al-Masjid). If he sits before praying Tahiyat al-Masjid, he does not commit a sin, because the command for it is for recommendation, not obligation.