What is the ruling on the prayer of those whose Imam forgot and did not sit for the middle Tashahhud after the second prostration in the second rak'ah of the 'Isha prayer, then he became aware due to the congregants saying "SubhanAllah," so he returned from standing to prostrating and completed his prayer?

If someone forgets the first tashahhud and only remembers it after standing up completely, they should not return to a sitting position, whether they have started reciting or not. This is based on what Al-Mughirah ibn Shu'bah narrated, that the Prophet, peace and blessings be upon him, said: "If one of you remembers before he stands up completely, let him sit; but if he has stood up completely, let him not sit." And if someone returns to a sitting position, knowing that their action invalidates the prayer, their prayer becomes invalid. However, if they were ignorant, their prayer remains valid. They should perform the prostration of forgetfulness (sujud as-sahw). The congregants must follow the imam in his standing, but they should not follow him if he returns [to a sitting position] after having stood up. If they follow him out of ignorance, their prayer is valid.