Mukhtasar Muslim

Sayyiduna Abu Sa'id Khudri (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: When any one of you is uncertain in his prayer (about how many rak'ahs he has prayed) and does not know whether he has prayed three or four, then he should dispel the doubt and act upon what he is certain of. Then, before saying the salam, he should perform two prostrations. Now, if he has prayed five rak'ahs, these two prostrations will make it six rak'ahs, and if he has completed four, then by these two prostrations dust will be thrown in the face of Satan.