- An addition is an addition to the prayer, and the prostration of forgetfulness is likewise an addition. It is therefore wise to defer the prostration of forgetfulness until after the salām, so that two additions are not combined within the prayer.
Thus, both the textual evidence of the Sunnah and the underlying rationale establish that prostration for an addition is performed after the salām.
“If he realizes it during it, he should sit down immediately, recite the tashahhud if he has not already done so, prostrate, and give the salām.”
His statement, “If he realizes it during it,” means: if he realizes the addition during the extra rak'ah.
His statement, “he should sit down immediately,” means: as soon as he realizes it, without delay. Thus, even if he remembers while bowing that this is a fifth rak'ah, he must sit down. Some students of knowledge may mistakenly think that this case is like that of someone who rises after the first tashahhud. They imagine that, once he has risen for the additional rak'ah and begun recitation, it becomes prohibited for him to return. This is an error and a misconception. One may never continue with an additional rak'ah; whenever he remembers, he must return in order to prevent this addition. If he were to continue with the addition while knowing of it, he would be deliberately adding something to the prayer, which is not permissible and invalidates the prayer.
His statement, “recite the tashahhud if he has not already done so,” means that if he realizes the addition and sits down, he should recite the tashahhud—unless he had already recited it before standing for the additional rak'ah. But is it possible for someone to add a rak'ah after having recited the tashahhud?
The answer is yes. For example, he may recite the tashahhud in the fourth rak'ah, then forget and think that it was the second; he then stands for what he thinks is the third rak'ah, but remembers after standing that this is the fifth rak'ah and that the tashahhud he recited was the final tashahhud. Thus, the author's statement, “if he has not already recited the tashahhud,” has a sound meaning.