If a person prays Dhuhr three rak'ahs by mistake, should he complete the fourth rak'ah and perform the prostration of forgetfulness (sujud al-sahw), or should he repeat the entire prayer?

If the worshipper remembers that he missed one or more units (rak'ahs) of prayer shortly after (around five minutes) while still in his prayer area or the mosque, he should complete the prayer by performing what he missed, then say the tasleem, prostrate for forgetfulness (sujud al-sahw), and say the tasleem again. However, if he remembers after a long interval, such as half an hour, or after leaving the mosque, he must repeat the entire prayer due to the lack of continuity.