The ruling on one who forgets a rak'ah and does not remember it until after the taslim

Question:
If I pray three rak'ahs in my four-rak'ah prayer, and after three rak'ahs I say the taslim, and after my taslim I remember that I only prayed three, and I stand up and perform one rak'ah, what is the ruling on my prayer?

Answer:
It is valid, and you must perform the prostration for forgetfulness. You must prostrate two times after the taslim; this is the recommended practice after the taslim. Pray one rak'ah to complete your prayer, then say the taslim, then prostrate two times for forgetfulness to make up for your prayer. This is because it is established that the Prophet (peace and blessings be upon him) prayed the 'Asr prayer in three rak'ahs on some days, said the taslim, and then was reminded. He then performed the fourth rak'ah, said the taslim, and then prostrated two times for forgetfulness after the taslim, just as happened to you.
Likewise, if you say the taslim from two rak'ahs of a four-rak'ah prayer, or from the Maghrib prayer, and then you remember after the taslim, you stand up and complete your prayer, then say the taslim, and then prostrate for forgetfulness. And if you are the imam and they do not remind you, they also stand up with you to complete the prayer, and then prostrate with you for forgetfulness after the taslim, just as happened to the Prophet (peace and blessings be upon him). Yes.

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