If a worshipper mistakenly stands for a fifth rak'ah after having recited the second tashahhud, is it obligatory to repeat the second tashahhud, or should he sit and say the taslim immediately upon remembering?

If a worshipper stands for an extra rak'ah inadvertently in a four-rak'ah prayer after completing the second tashahhud, then remembers, he must immediately return to a seated position and say the tasleem, then perform the prostrations of forgetfulness (sujud al-sahw). He does not need to repeat the second tashahhud because he had already performed it before standing for the extra rak'ah.