Another example: if a person omits al-Fātiḥah, the prostration of forgetfulness is required of him. However, he must do something else in addition to the prostration of forgetfulness: he must perform the omitted pillar. What one should do when omitting a pillar has already been explained. (1)

A third example: if he forgetfully omits the first tashahhud, he is only required to prostrate; he is not required to perform it, because it is an obligation that is waived in the case of forgetfulness.

A fourth example: if he omits the opening supplication, the prostration of forgetfulness is not required of him, because his prayer would not be invalidated if he deliberately omitted it.

But is it recommended?

The correct view is that, if he omits it forgetfully, prostration is recommended, because it is a legislated utterance and may therefore be compensated for by the prostration of forgetfulness. Yet the prostration of forgetfulness is not obligatory, because that for which the prostration is performed is not itself obligatory; thus, what follows from it is not obligatory either. Therefore, if a person forgetfully omits a sunnah that he normally performs, the prostration of forgetfulness for it is sunnah. But if he deliberately omits the sunnah, then prostration is not prescribed for him, since its cause—namely, forgetfulness—is absent.

His statement, “that whose deliberate performance invalidates it”: the word “that” here is a relative pronoun, and thus encompasses both doing and omitting. So, if one adds an extra bowing out of forgetfulness, prostration is obligatory upon him, because if he were deliberately to add a bowing, his prayer would be invalidated.

If he performs a legislated utterance in other than its proper place—for example, if he recites while sitting, having forgotten—then prostration is not obligatory upon him, because his prayer would not be invalidated if he deliberately recited while sitting.