What is the ruling regarding a congregant who performed the prostration of forgetfulness (sujud al-sahw) thinking that the imam had done so, then completed his prayer, performed a new rak'ah, and performed the prostration of forgetfulness?

The worshiper who missed the bowing (ruku') must perform it and then follow the Imam. He is not required to perform the prostration of forgetfulness (sujud as-sahw) because the Imam bears it on his behalf. If he did not perform the bowing and followed the Imam, he must perform an additional rak'ah to replace the one whose pillars he partially omitted. His prayer is valid, and he is not required to perform the prostration of forgetfulness.