The ruling on one who prostrated for forgetfulness thinking it was the imam’s prostration, then realized he was standing

Q: We were praying the Maghrib prayer in congregation, and during the final tashahhud in the third rak'ah, the imam said takbir and stood up intending to perform a rak'ah. Some of the worshippers did not notice the imam standing up, so they prostrated, assuming that the imam had said takbir for the prostration of forgetfulness. When they raised their heads from the prostration, they noticed the imam sitting down because he had heard the recitation of "Subhan Allah." The imam then prostrated twice, and after the taslim, it became clear to some of the worshippers that they had prostrated three times. What is the ruling on the prayer in this case? And what is the ruling on the third prostration for some of the ma'mum?

A: There is no blame on one who prostrated thinking it was the imam’s prostration for forgetfulness, and his prayer is valid; because he did not intentionally add to the prayer, but rather prostrated in following the imam according to his belief.