Must one make up the prayer if the number of rak'ahs was not determined—whether it was the third or fourth—and then an additional rak'ah was completed and the prostration of forgetfulness was performed, knowing that it is obligatory to complete an additional rak'ah and then perform the prostration of forgetfulness?

If a worshipper doubts during their prayer, either one of the two possibilities will be more probable in their estimation, or both possibilities will be equally probable:

- If one of the two possibilities becomes more probable to them, they should act upon what is more probable and complete their prayer, then perform the prostrations of forgetfulness (sujud al-sahw) after the salutations (tasleem), based on the Prophet’s (peace be upon him) saying: «WHEN ONE OF YOU DOUBTS IN HIS PRAYER, HE SHOULD STRIVE TO DISCERN WHAT IS CORRECT, THEN COMPLETE HIS PRAYER ACCORDINGLY, THEN PRONOUNCE THE SALUTATIONS, THEN PROSTRATE TWICE.»

- If neither of the two possibilities becomes more probable to them, they should act upon certainty, which is the lesser number, and build upon it, complete their prayer, and perform the prostrations of forgetfulness before the salutations, based on his (peace be upon him) saying: «WHEN ONE OF YOU DOUBTS IN HIS PRAYER, AND DOES NOT KNOW HOW MANY (RAK'AHS) HE HAS PRAYED? THREE OR FOUR? HE SHOULD DISCARD THE DOUBT AND BUILD UPON WHAT IS CERTAIN (THE LESSER NUMBER), THEN PROSTRATE TWICE BEFORE PRONOUNCING THE SALUTATIONS.»

However, if the doubt is merely a whisper (waswasah) or occurs after the completion of the act of worship, it should not be paid attention to, and no prostration of forgetfulness is required for it.

And if the worshipper acts upon the weaker possibility, or if the doubt was equally probable and they acted upon the greater number of rak'ahs, then the prayer is invalid. Most scholars are of the opinion that it is obligatory to repeat it. Sheikh al-Islam Ibn Taymiyyah chose the view that repeating it is not obligatory if the person was ignorant of the ruling, unless they learned of it before the prayer time expired.

It is safer to repeat the doubtful prayer, because it is a single prayer and repeating it is not difficult.