If a person falls into doubt as to whether he prayed, e.g., three of four raka‘at, he should consider the most likely case. However, if he cannot put more weight to one of the two, he should assume what he is certain of and that is lesser amount, and make the prostrations of forgetfulness (sujood ul-sahw). The evidence is the hadith narrated by Abu Sa‘eed Al-Khudri (may Allah be please with him), who said:The Prophet (peace be upon him) said: If one of you has doubts during his prayer and he does not recall how many rak‘at he has prayed, whether three or four, then he can put an end to his doubt by praying according to what he was certain of (the lesser amount) and then making two sujood before the salaam. If it turns out that he had prayed five raka‘at, the two sujood would make it even. Whereas if he ended up completing his four raka‘at , they would be in defiance of the Satan.
(Sahih Muslim, No. 571)