What is the ruling on the ma'mum not following the imam in the prostration of recitation?

Question:
The questioner says: I prayed the 'Isha' prayer in one of the mosques, and the imam recited a surah containing a prostration. When the imam reached the prostration, he said takbir and bowed, and some of the worshippers prostrated because they did not see the imam. When the imam raised his head from the bowing, they realized that he had not prostrated, and they did not perform a rak'ah. Is their prayer valid, or do they have to perform a rak'ah?

Answer:
It is obligatory for them, if they raised their heads and knew that he had not prostrated, to bow, then bow again, then follow him, and that suffices them. If they raised their heads when he said, "Allah hears the one who praises Him," they bow, then raise their heads, then follow him, and that suffices them. As for if they followed him in the prostration and did not bow, they have to perform a rak'ah. If they were ignorant of the matter, when he says the closing salam, they perform a rak'ah to complete their prayer. But if they were deliberate and playing with it, their prayer is invalid. However, the common case is that this is due to ignorance, so when he says the salam, they perform the rak'ah they missed; they stand up and perform it. But it is obligatory for them, when he says, "Allah hears the one who praises Him," to stand up and be upright, then bow, then raise their heads, then catch up with him in the prostration. This is what is obligatory. Yes.