If the one who prays remembers a mistake after the taslim, does he prostrate for it? And is the tashahhud prescribed after the prostration for forgetfulness?

The questioner : A question regarding the prostration for forgetfulness, if one remembers the mistake after the taslim?

The Shaykh : He prostrates, even if it is after he has left.

The questioner : And in that case, the matter requires greetings and taslim.

The Shaykh : It varies according to the schools of jurisprudence. What is established in the Sunnah, regardless of the specific situation you are asking about, is that it is attached to the rule that the prostration for forgetfulness does not require a tashahhud. Do you understand? That is, he prostrates directly, either before the taslim or after the taslim.

The questioner : ... meaning, the usual ones.

The Shaykh : There is no need for a second tashahhud. So in all the cases, including the one you are asking about, there is no tashahhud.

The questioner : And no taslim.

The Shaykh : No, and no taslim, meaning the first one.

The questioner : No ... the two prostrations.

The Shaykh : It is necessary. This leaving is necessary.

The questioner : Leaving the prayer?

Another questioner : The question you raised.

The Shaykh : Leaving the prayer—may Allah bless you—if there is no prostration for forgetfulness, this is a pillar. When he has a prostration for forgetfulness, he is given the choice between performing the taslim, which is a pillar, and delaying it. So if he performs the taslim and then prostrates the two prostrations for forgetfulness, he must perform the taslim again. Yes. And if he did not perform the taslim, that is also permissible. That is why I said: if he wishes, he prostrates after the taslim, the two prostrations for forgetfulness; and if he wishes, he prostrates before the taslim. If he did not perform the taslim the first time, or if he did perform the taslim the first time, he must perform the taslim the second time; that is, after the two prostrations.

The questioner : But the greetings.

The Shaykh : The greetings are not prescribed. There is no authentic hadith, or more than one hadith, regarding this. It came in Sahih al-Bukhari and others that the Messenger (peace be upon him) did not perform the tashahhud again after the two prostrations for forgetfulness. So the situation you are describing falls under the general rule. And especially, as I said, even if he has left. You said that he performed the taslim and then remembered. I added to that: even if he has left. And I will add a second, more emphatic point: even if he has walked away and departed. This happened to the Messenger (peace be upon him). He prayed Maghrib once with two rak'ahs, and the people stood up and left. Some of them said, "O Messenger of Allah, did you pray only two rak'ahs?" He ordered the iqamah to be called again, the iqamah for the prayer, so that the people who had rushed to leave would hear. So he established the prayer and came back for the rak'ah he had forgotten. Then he performed the taslim of leaving the prayer.
The important thing is that whenever he remembers, at whatever time he remembers, he must perform this obligation. As for if he intentionally leaves the prostration, he is sinful, and there is no way to make up for what he neglected on his own.