What is the ruling on the prayer of one who stood up to make up what he missed, thinking that the imam had said the closing salam, but then it became clear to him that the imam had not?

The questioner : Shaykh, may Allah reward you: What is the validity of the prayer of one who caught up with the imam for one unit of prayer on the roof during the final unit, and thought that the imam had said the closing salam, so he stood up to make up what he missed, but then it became clear to him that the imam had not said the salam? Does that unit count for him, given that he was on the roof and did not hear the imam’s closing salam?

The Shaykh : Was there anyone with him or not?

The questioner : Was there anyone with him? Yes.

The Shaykh : Then what is the need for him to be on the roof? Even if the mosque is large and he is in the last row, meaning a man stood up to make up what he missed, thinking that the imam had said the closing salam, but then it became clear that the imam had not. If the one who joined the prayer late stands up to make up what he missed, thinking that the imam had said the closing salam, but then it becomes clear that the imam had not, he must go back. When the imam says the closing salam, he stands up, makes up what he missed, and then performs the prostration for forgetfulness after the salam.