If the Worshipper is in Doubt over how many Rak'ahs He has prayed

Question :

I have read in some Islamic law books that when prayer is made and the worshipper is in doubt about the number of Rak'ahs, it is invalid. And in some books, it says if the worshipper is in doubt, he should prostrate twice after completing the prayer. Which is correct?

Answer:

What is correct is that the prayer is not invalidated, because such doubts often come to a person without his desiring it, and the Prophet made clear the ruling on one who is doubt regarding his prayer, and that doubt falls into two categories:

The first category is: When a person is in doubt concerning the number of Rak'ahs he has prayed, although he is surer of one than the other. In this situation, a person should base his assumption on what he is most sure of, then complete the prayer and then make the Taslim, then make the prostration of forgetfulness after the Taslim.

The second category is: When a person is in doubt over the number of Rak'ahs he has prayed and he is unable to reach any conclusion about which is the more likely number. In this situation, he must assume the lesser number, because he is sure of that, and he is in doubt about the extra, so he should complete his prayer based upon the lesser number. Then he performs the prostration of forgetfulness before the Taslim and his prayer will not be invalidated by this. This is the ruling on doubt in the number of Rak'ahs prayed.

Likewise, if he is in doubt over whether he has made the second prostration or not, or whether he has performed the bowing or not, if he is more sure of one than the other, he must complete his prayer based upon what he is more sure of and then perform the prostration of forgetfulness after the Taslim.

If he is completely unsure which of the two numbers he has performed, then he should proceed based upon what is safest and assume that he has not performed this bowing or this prostration about which he is in doubt. So he should perform it and what comes after it, and complete his prayer based upon this assumption, then perform the prostration of forgetfulness before making the Taslim. But if he reaches the point in the prayer of which he is in doubt (i.e. in the next Rak'ah), then the following Rak'ah will be considered to take the place of the one he left out of that (essential) pillar.

Source:
Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 2 Pages 275-276