What is the ruling on the prayer of one who, during the last Tashahhud, doubted whether he had performed the sitting between the two prostrations, then changed his intention to rectify that, then returned to the Tashahhud, and performed the prostration of forgetfulness at the end of the prayer? Does the prayer become invalid thereby, and what is incumbent upon him if five days have passed since this prayer?

If someone doubts having omitted a pillar of prayer, they must make it up. If they remember it before the opportunity to make it up passes, they must perform it and whatever comes after it, then prostrate for forgetfulness (sujud al-sahw). If the opportunity to make it up has passed, the rak'ah (prayer unit) becomes invalid, and the subsequent rak'ah takes its place.

So, if a worshipper remembers omitting the sitting between the two prostrations while they are in the tashahhud, they should, while still sitting, intend to perform the sitting between the two prostrations, then prostrate, then repeat the tashahhud from the beginning, complete their prayer, and then perform the prostration for forgetfulness. If they do not do so, their prayer is invalid and they must repeat it.

As for someone suffering from obsessive doubts (waswas), they should not pay attention to their doubts. Instead, they should proceed with their prayer and consider what they have done as correct.