Sujood ul-sahw (prostration for forgetfulness) if one 
joins congregation late
Question

                What about someone who joins the congregational 
prayer late and has to make up the rak’aat missed, and 
finds that the Imam makes sujood al-sahw? Should he 
join in the sujood or stand and continue his prayer and 
make it at the end?
Praise be to Allah.If the Imam makes the salaam and the follower stands up to 
complete what he missed, while the Imam has to make prostrations 
of forgetfulness after the salaam, the follower should return back 
and make prostrations with the Imam if he has not completed 
standing upright. Otherwise he should not return. He should 
complete his prayer and after it he should make the missed 
prostrations. The evidence for this is the same as presented in the 
discussion for Forgetting to sit for tashahhud between the second 
and third raka’ah. (al-Mughni 1/697)