If a worshipper stands for the fourth rak'ah and does not sit for the final tashahhud, and then remembers or is reminded by someone, should he return to a sitting position, or should he complete it and pray a fifth rak'ah, then perform the two prostrations of forgetfulness (sujud al-sahw)?

If a worshipper inadvertently adds to their prayer, such as standing for a fifth rak'ah in a four-rak'ah prayer, they must return (to the correct position) as soon as they remember the addition. Then, they should sit, recite the tashahhud, say the taslim, and perform the prostrations of forgetfulness (sujud al-sahw). It is not permissible for them to continue with the addition after becoming aware of it. If they do so, their prayer becomes invalid.