In sum, one view is that he must repeat the prayer if he deliberately omits these prostrations; another is that he must repeat it whether he omits them deliberately or forgetfully. The correct view is that he must either perform these prostrations or repeat the prayer. There is disagreement over how long afterward he may still perform them.

One view is that he may perform them so long as he remains in the mosque, provided the interval is not prolonged. Another is that he may perform them even after a prolonged interval, so long as he remains in the mosque. A third is that he may perform them even after leaving and going farther away. The point is that he must either perform them or repeat the prayer, because they are obligatory: the Prophet, peace and blessings be upon him, commanded them to complete the prayer. A person’s obligation is therefore not discharged without them. If he commands them after the prayer’s closing salutation, and one is told, “Either perform them or you must repeat the prayer,” there is nothing untenable in this.

To say that the prayer is invalid means that it has not discharged one’s obligation. In this respect, there is no difference between prostrations prescribed before the closing salutation and those prescribed afterward. Allah Most High has permitted ending the prayer with the salutation only on condition that one performs the two prostrations of forgetfulness. If one does not perform them, He has not permitted one to leave the prayer. One has thus ended the prayer with a salutation that was not commanded, thereby invalidating it.

Similarly, we say of someone who converts his Hajj into an ‘umrah to be followed by Hajj: he is permitted to leave ihrām (ritual consecration) only if he intends to do so and perform Hajj that same year. If, however, he intends simply to leave ihrām, he is not entitled to do that. He remains in ihrām, and his departure from it is not valid. Yet ihrām is not ended by a person’s repudiating it, committing any of the acts prohibited during it, or vitiating it. Rather, he remains in it even when it has been vitiated. Prayer is different: it is invalidated by doing something incompatible with it or prohibited during it.