Saying takbirat after the imam is a sunna

Assalamu 'alaykum…I did not know that we are supposed to say takbeer whilst praying behind an imam, is my prayer invalid or is it valid since I did not know? I did say the opening takbeer but I remained silent for the takbeerat that followed. Jazakallahu khayr. I also came home after I found out about this and did sajda sahw.

Alaikumassalam wa rahmatullah,

Saying the takbirs other than the first one (known as takbīrāt al-intiqāl) is a sunna, not a fard, therefore there is no issue with you not having said them (although a person should not intentionally abandon them). Also note that these takbīrāt al-intiqāl should be said silently (in a whisper), only the imam says them aloud.

References:

Fatwa from the Qatari Fatwa Authority (Arabic PDF)