Question:

If someone misses a wajib (obligatory) aspect of Salaah while he is in congregation, what should he do?

Answer: 

The fatwa of majority of the scholars of our time is that if he forgets one of the waajibaat (obligatory acts) while he is being led by the imaam, then he doesn’t do sujood sahw (prostration of forgetfulness) due to the hadeeth “و الإمام ضامن”. So, they took from it that the imam bears for you such short comings in the prayer and Allah knows best.

~ Abu Fajr AbdulFattaah bin Uthman