If a muqtadi misses a fardh action behind the Imam, does he have to repeat the salah? For example, missing a sajdah [prostration].
بِسْمِ اللهِ الرَّحْمنِ الرَّحِيْم
In the name of Allah, the Most Gracious, the Most Merciful
Imam Quduri explains that there are six requirements of prayer, and one of them is prostration. If the prostration is skipped, the prayer will not be valid and must be repeated. However, if the follower [muqtadi] is a little late but still performs the prostration, it will not count as being missed, and the prayer will be valid.
Only Allah knows best
Written by Maulana Mustafa Umar
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham
 See Mukhtasar Al-Quduri p. 33, Taha Publishers, 201