ANSWER:

As long as the imam & the follower were in ruku position at the same time, even for a second, the rakah is valid for the follower. If he missed it and Imam rose before he went to ruku with him, he has to make up for that missed Rakah.

QUESTION: What is the validity of a rakah for a follower of an imam when the follower goes into ruku behind the imam whilst the imam is seen rising out of ruku or saying the dhikr between ruku and standing up? Assume the follower had already started the salah with the imam and rakah is quiet.