The Prophet (ﷺ) said: "If one of you does not know whether he prayed three or four (rak'ahs), let him pray a rak'ah then prostrate twice after that when he is sitting. Then if he prayed five (rak'ahs), they (the two prostrations) will make his prayer even-numbered, and if he had prayed four, they will annoy and humiliate the shaitan."
Sunan an-Nasa'i Book 13, Hadith 61
← Previous Hadith The Prophet (ﷺ) said: "If one of you is not sure about his prayer, let him forget about what he is unsure of and complete his prayer on the basis of what he is sure of. When he is sure that he has completed it, let him prostrate twice while he is sitting. Then if he has prayed five (rak'ahs), they (the two prostrations) will make his prayer even-numbered, and if he had prayed four, they will annoy and humiliate the shaitan."

Sunan an-Nasa'i Book 13, Hadith 60 Next Hadith → "If one of you is not sure about his prayer, let him estimate what he thinks is most likely to be correct and complete the prayer on that basis, then let him prostrate twice."

Sunan an-Nasa'i Book 13, Hadith 62