What is the ruling on one who was late to the prayer by one unit of prayer, forgot, and then remembered after the closing salam?

The questioner : I was late to the prayer, and when the imam said the closing salam, I forgot that I was late. I was about to say the salam with him, but when I turned to my neighbor, who was also late, I saw that he was about to stand up. I then remembered and stood up. Does this count as a mistake, and does it require the prostration for forgetfulness?

The Shaykh : This issue has two aspects. If you made the mistake before you separated from the imam, then there is no prostration for forgetfulness required of you. Otherwise, the prostration for forgetfulness is required of you.

The questioner : Yes, may Allah reward you with good.

Another questioner : ... .

The questioner : It is the same after the imam has said the salam. We know that it is a single salam that ends the prayer.

The Shaykh : Yes, exactly.