It is mandatory to pray Tahiyyat al-Masjid (two rak’ahs “greeting the
mosque”); indeed, this is obligatory, even if the Azaan is being called or
khateeb is on the minbar giving the khutbah.
This is because of the hadeeth of Jaabir ibn ‘Abd-Allaah who said: a man
came whilst the Prophet (peace and blessings of Allaah be upon him) was
addressing the people on the day of Jumu’ah. He said, “Have you prayed, O
So and so?” He said, “No.” he said, “Stand up and pray two rak’ahs.”
(Narrated by al-Bukhaari, 888; Muslim, 875).