The imam added a rak'ah and we alerted him, but he did not return, so I sat until he completed the taslim, and the rest followed him. What is the ruling on my prayer and their prayers?

Question:
He says: Honorable Shaykh, may Allah grant you success. The imam prayed 'Isha' with us in five rak'ahs, and we alerted him, but he did not return. I sat and did not follow him until he completed the taslim with me, and the rest of the ma'mum followed him. What is the ruling on my prayer and their prayers?

Answer:
Your prayer is valid, as long as you knew that it was an extra rak'ah. It is not permissible for you to follow him. Sitting is obligatory for you. You sit, complete the taslim with him, and do not complete the taslim on your own. As for the others who followed him, if they were ignorant, their prayer is valid, and they are excused due to ignorance. However, if they did it intentionally, their prayer is invalid, if they knew that it was an extra rak'ah and knew that it is not permissible to follow him. Yes.