Does the prayer of one who is a rak'ah behind the imam and then forgets to make up that rak'ah become invalid?

The questioner:
The questioner says: A person joined the 'Isha' prayer and had missed a rak'ah. When the imam said the taslim, he said it with him out of forgetfulness. How does he make up what he missed? Does he repeat the prayer?

The Shaykh:
A man.

The questioner:
A person joined the 'Isha' prayer and had missed a rak'ah. When the imam said the taslim, he said it with him out of forgetfulness. How does he make up what he missed? Does he repeat the

The Shaykh:
If he said the taslim out of forgetfulness, he stands up and performs the remaining rak'ah, then performs the prostration for forgetfulness, and he does not repeat the prayer. If he said the taslim out of forgetfulness, he owes nothing, for forgetfulness is excused for the Ummah, as are error and forgetfulness. He must hasten to stand up and perform what remains of his prayer, then perform the prostration for forgetfulness regarding the taslim that occurred from him. Yes.