The ruling on the prostration for forgetfulness for the ma'mum if he prostrates with his imam

Question : He says: If the ma'mum forgets with his imam, he forgets with his imam, and something is missed from him. And he prostrates with the imam the prostration for forgetfulness, is it obligatory for him to prostrate when he has finished completing his prayer for his forgetfulness, or is it not obligatory?

Answer : We say it is obligatory, because his prostration with the imam was only following the imam, and he was in the middle of the prayer. And the prostration for forgetfulness by which the prayer is made complete does not occur except either after the taslim or after the tashahhud at the time of the taslim.