Massallah pertaining to reading namaaz in Jamaah

assaslaam-u-alikum mufti saab
i have recently started paraying namaz 5 times daily(last week)
for 2 days i have been praying zuhr, and asr and maghrib and isha behind my local imam.
when praying asr and zuhr behind an imam, the imam is silent all throughout the prayer except for reciting allah-ho-akbar, in the final two rakats i recite thana
but in the first two rakaats i dont know what to recite?