What is the ruling on doubting the number of prostrations in the first rak'ah while prostrating, then having a stronger feeling that they were completed, then becoming certain of the deficiency in the second rak'ah? And is it correct to cancel the first rak'ah, complete the prayer, and then perform the prostration of forgetfulness (sujud al-sahw) after the tasleem? And what is the ruling on someone who doubted saying "Rabb ighfir li" (Lord, forgive me) between the two prostrations in the Maghrib prayer, and did not perform the prostration of forgetfulness due to not knowing the ruling? And is their prayer valid, and what are the implications, given that they performed the prostration of forgetfulness after another prayer?

The questioner should have performed the second prostration before moving to the next rak'ah. Her action of building upon certainty and performing an additional rak'ah instead of the one she suspected was missing a prostration, followed by the prostration of forgetfulness (sujud al-sahw), was correct. The fundamental principle is to build upon certainty when in doubt about omitting a pillar. As for the doubt regarding saying the du'a between the two prostrations, if one holds it to be obligatory, then it should have been performed if it was possible to rectify it. If it was missed, it does not affect the validity of the prayer, nor does it necessitate the prostration of forgetfulness, because it is a doubt concerning the omission of a wajib (obligatory act). These rulings apply to most people. However, for those who frequently experience doubts and waswas (obsessive whispers), they should disregard their doubts and build upon what is most likely.