When the Imam is in Doubt over whether He has prayed Three or Four Rak'ahs

Question :

A Muslim brother is asking: If a person is in doubt during his prayer and he does not know how many Rak'ahs he has prayed, four or three, is it correct for him to repeat the prayer? I request an explanation - thank you.

Answer:

If a person is in doubt regarding his prayer, and he does not know how many Rak'ahs he has prayed, whether three or four, it is not permissible for him to leave his prayer due to this doubt, if the prayer is an obligatory one, because it is not allowed to break off an obligatory prayer. He should act in accordance with the Sunnah; and according to the Sunnah, if a person is in doubt regarding his prayer and he does not know how many Rak'ahs he has prayed, whether three or four, he must be in one of two situations: The first is that he is in equal doubt, that is, he is equally unsure whether he has prayed three or four; in this case, he must assume the lesser amount (three), then pray the fourth and prostrate due to forgetfulness before making the Taslim.

The second situation is when his doubt is more inclined towards one possibility than the other, that is, he is in doubt over whether he has prayed three or four, but he feels it is more likely that he has prayed four; in this situation, he should assume that he has prayed four, then make the Taslim and then perform the prostration of forgetfulness after the Taslim.

This is what the Sunnah says regarding the difference between the two situations in which doubt occurs. The Prophet ordered us to base our assumption on what we are sure of in the first situation and to try to ascertain the truth in the second situation, which proves that one should not leave the prayer due to doubt. If it is an obligatory prayer, leaving it is prohibited, because breaking off an obligatory act of worship is forbidden. And if it is a non-obligatory prayer, he should not leave it because of this doubt, but he should do as the Prophet ordered. However, if he wishes, he may break off this prayer, because breaking off a non-obligatory act of worship is permissible, although the scholars say that doing so is disliked, unless there is a valid reason for it. That is, if the non- obligatory act is not Hajj or 'Umrah; if it is Hajj or 'Umrah it is not permissible to break it off unless it is due to some impediment, as Allah, the Most High says:
And perform properly (all ceremonies according to the ways of Prophet Muhammad) the Hajj and Umrah (i.e. pilgrimage to Makkah) for Allah. But if you are prevented (from completing them), sacrifice a Hadi (animal, i.e. a sheep, a cow or a camel) such as you can afford.
And this Verse was revealed before the Hajj was made obligatory. It was revealed during (the Treaty of) Al- Hudaibiyyah and Hajj was made obligatory in the ninth year. And Allah knows best.

Source:
Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 2 Pages 276-277