The completion of obligatory deeds (1) by voluntary deeds applies without qualification, for it will take place on the Day of Resurrection, the Day of Recompense. If a person neglects some obligatory duties, he deserves punishment; but if he has a voluntary deed of the same kind (2), it will take their place and he will not be punished. And if (3) his reward is deficient and he has a voluntary deed, it will make up the deficiency and complete his reward.
In this world, however, he is instructed to repeat an act that he performed (4) deficiently in respect of its obligatory elements (5), whenever it is possible to repeat it, or to rectify it through the prescribed means of compensation. Examples include the two prostrations for forgetfulness in prayer, the compensatory sacrifice for an obligatory rite of Hajj that he omitted, and the alms of fast-breaking, which were prescribed to purify the fasting person from idle and obscene speech. This is because, if he is able (6) to perform the obligatory act, it remains incumbent upon him: he has not discharged his obligation, but is still required to perform it (7), just as if he had not performed it at all. This is unlike the situation when its performance becomes impossible on the Day (8) of Recompense, for nothing remains there except good deeds.
For this reason, the majority of scholars hold that whoever omits one of the obligatory acts of prayer