Sayyiduna Abu Sa'id Khudri (may Allah be pleased with him) narrates that the Noble Prophet (peace and blessings be upon him) said: If anyone is uncertain in prayer whether he has prayed three (rak‘ahs) or four, he should stand up and pray one more rak‘ah, so that the doubtful rak‘ah becomes an extra one. Then, before the salam, he should perform two prostrations of forgetfulness (sujud al-sahw). If he has prayed five rak‘ahs, these prostrations will make them even, and if he has prayed only four, they will humiliate Satan.