In the case of being uncertain, such as if a person is not sure if he has done something extra or omitted something, e.g., he is not sure whether he has prayed three rak’ahs or four, one of the following two scenarios must apply:
Either he thinks that one or the other is more likely to be case, whether it is doing more or omitting something, in which case he should proceed on the basis of what he thinks is more likely to be the case, complete his prayer accordingly, then do the prostration of forgetfulness after saying the salaam;
Or neither appears more likely to be the case, so he should proceed on the basis of what is certain, namely the lesser amount, and complete his prayer accordingly, then do the prostration of forgetfulness before saying the salaam.
For example: a man prays Zuhr then he is not sure whether he is praying the third or fourth rak’ah, but he thinks it more likely that it is the third. So he should pray another rak’ah, then say the salaam, then do the prostration of forgetfulness.
An example of when neither seems to him more likely to be the case is when a man is praying Zuhr and is not sure whether he is in the third rak’ah or the fourth, and neither seems to him more likely to be the case. In that case he should proceed on the basis of what is certain, which is the lesser amount. So he should regard it as the third rak’ah, then do another rak’ah and do the prostration of forgetfulness before saying the salaam. Thus it becomes clear that this should be done before the salaam if you have omitted one of the obligatory parts of the prayer or if you are not sure how many rak’ahs you have done, and neither of the two choices seems more likely to be the case. And it should be done after the salaam if you have added something extra to the prayer or you are not certain but one of the two choices seems more likely to be the case.
See Majmoo’ Fataawa al-Shaykh, 14/14-16
And Allaah is the Source of strength.
