Bismillahi Al-Rahmani Al-Rahim
It is fardh to say takbir from position to position during salah. If one is praying behind an Imam obviously one should repeat the takbir silently after the Imam. However, what should you do if you forget to say Allahu Akbar as you move from, say, sajdah to standing? Should you do the sajdah sahu after the Imam has said salams?
Saying "allahu akbar" is obligatory [wajib, not fard] for the imam and for individuals. It is not obligatory to say "allahu akbar" when praying behind an imam, though it is recommended. Even if one were to intentionally omit saying "allahu akbar"-with the exception of the inaugural takbir-when praying behind an imam, it would have no effect on the validity of the prayer, and there is absolutely no reason to make sujud sahu for doing so.
And Allah knows best.
Wa al-salamu `alaykum,