Is it valid to cancel a rak'ah and return to it when in doubt about the rukoo' (bowing) after descending for sujood (prostration), and is it obligatory to repeat the prayers in which this occurred?

If a worshiper doubts, while performing the prayer, whether he has omitted a pillar of the prayer, he should act based on certainty and consider that he has not performed it, then he should make it up. If he is unable to do so, he should nullify the rak'ah (unit of prayer) in which he doubted a pillar was missing and perform another rak'ah in its place. This ruling applies to normal doubt, not to obsessive whispering (waswasah). Doubt that occurs after completing the act of worship is not given any consideration.