The prayer of one who adds a fifth rak'ah to the 'Isha' prayer

The questioner:
The 'Isha' prayer, then he stood up for the fifth, thinking it was the fourth. Yes. So he was certain. Yes. When the believers recited the tasbih, but he did not sit. Yes. So the people stood up with him. The fifth rak'ah, and they knew it was the fifth. When the imam was asked, he meant that one should eat the 'arifa, and he meant it did not mean.

The Shaykh:
He is excused.

The questioner:
So he informed them that whoever adds to the fourth rak'ah intentionally, his prayer is invalid.

The Shaykh:
The prayer is invalid with two conditions: that he did it intentionally and that he knew the ruling. He must know the ruling. But if he was ignorant or did not do it intentionally, his prayer is not invalid. So if they were ignorant of the ruling, their prayers are valid, if Allah wills.

The questioner:
What does he do?

The Shaykh:
Hmm?

The questioner:
He waits, he sits, he waits.

The Shaykh:
He sits if he knows the ruling and knows that it is an extra rak'ah. He sits and recites the final tashahhud. If he wishes, he says the taslim for himself, and if he wishes, he waits for the imam and says the taslim with him.