How, then, could it be cited when his wording in the Sahīh entails that both are obligatory—the rak‘ah and the two prostrations?

The scholars agree that the rak‘ah is obligatory. Wherever it is held that someone in doubt must disregard his doubt and proceed on the basis of what he is certain of, the rak‘ah about which he is uncertain is obligatory. Since it is obligatory by the explicit text and scholarly agreement, and the reported wording concerns both it and the prostrations—which are likewise commanded—it follows that the wording cited does not negate the obligation to perform the two prostrations, any more than it negates the obligation to perform the rak‘ah.

If the Messenger did indeed say these words, their meaning is that the worshipper is commanded to do this when in doubt. If his prayer was actually complete, with nothing missing, this would constitute an addition to his deeds for which he would receive a reward, as with voluntary worship. This is the case with anyone who takes precautions by performing an act whose obligatory status he doubts: if it is obligatory, he has fulfilled it; otherwise, it counts as voluntary worship for him. Thus, he described it as voluntary in reality only on the supposition that the four rak‘ahs had already been completed. Yet the worshipper’s very doubt introduced a deficiency into his prayer, so he was commanded to perform the two prostrations even if he had prayed four rak‘ahs, to humiliate Satan.

This is like their instructing someone who has doubts concerning something other than the obligatory act to do what establishes that he has discharged his obligation: in reality, only one act is obligatory, and the additional act is voluntary. Likewise, someone who cannot distinguish his sister through breastfeeding from an unrelated woman is instructed to avoid both, although in reality only one is forbidden to him. Thus, the act in doubt is called obligatory in the sense that he is required