The ruling on completing the last rak'ah of 'Isha' after the imam has said the closing salam while in congregation

The questioner : Well, please. Today regarding the 'Isha' prayer, I arrived late by one rak'ah, O Shaykh? Yes, I was late by one rak'ah. I said the closing salam with them at the second salam. I remembered my mother. I still had one rak'ah left. So I said to myself, "Complete it." I stood up, completed it, and said the salam. Was that permissible? Yes, it is.

The Shaykh : It is permissible.

The questioner : May Allah keep you safe.

The Shaykh : And you, and upon you be peace and the mercy of Allah.