When is it prescribed for the worshipper to perform the prostration of forgetfulness in his prayer?

The prostration of forgetfulness (sujood as-sahw) was legislated to mend defects and deficiencies in prayer, and its causes are three: augmentation (such as an extra bowing or prostration), omission (such as omitting a pillar or an obligatory act), and doubt (such as hesitation about the number of rak'ahs).

If the worshipper intentionally adds something, their prayer becomes invalid. If they add something out of forgetfulness, they complete their prayer and perform the prostration of forgetfulness after the tasleem (salutation of peace).

If the worshipper omits a pillar, and remembers it before its place in the next rak'ah, they return and perform it and what comes after it, and then prostrate after the tasleem. If they do not remember it until its place in the next rak'ah, the subsequent rak'ah is considered a substitute for the one from which a pillar was omitted, and they perform an additional rak'ah, and then prostrate after the tasleem.

If the worshipper omits an obligatory act and proceeds to the next position, they complete their prayer and perform the prostration of forgetfulness before the tasleem.

Doubt is hesitation between augmentation and omission, and it has two cases: 1. If one of the two sides is more likely, one builds upon what is more likely and completes their prayer, then performs the prostration of forgetfulness after the tasleem. 2. If neither side is more likely, one builds upon certainty (the lesser number) and completes their prayer, then performs the prostration of forgetfulness before the tasleem.

The prostration of forgetfulness is performed before the tasleem in the case of omitting an obligatory act or doubting without one side being more likely. It is performed after the tasleem in the case of augmentation or doubting with one side being more likely.