The ruling on one who adds to the prayer intentionally or out of forgetfulness

If I entered the four-unit prayer with the imam in the first unit, and at the time of the closing salam I doubted whether I was missing a unit or not, so I stood up and performed a fifth unit, which I thought was the fourth for me, and after I finished the prayer, the person next to me told me that I had performed five units, so I prostrated for forgetfulness. Is my action correct?
Answer: As long as you know that you entered the prayer with him at the beginning, it is not permissible for you to stand up and perform a fifth unit; this action is invalid. It is not permissible for you, and it invalidates the prayer because you added to it intentionally. However, if you did that thinking that you had missed a unit, and then you learned afterwards that you had not missed anything, it does not harm you; the prayer is valid, and praise be to Allah. The point is that a person does not stand up to make up a unit unless he knows that he missed a unit, or he doubts whether he caught a unit or did not catch a unit. In that case, he performs a unit to complete his prayer and prostrates for forgetfulness because he has become alone in the last unit. But if he knows that he entered with... he has knowledge that he entered in the first unit, then he does not do anything; this is a whisper of Satan with no basis.
The presenter: May Allah reward you with good and do you good.