What should a person do if they stand up before performing the final sitting in prayer?

If someone performing prayer stands up out of forgetfulness before the final sitting, as long as they have not yet gone into prostration, they should sit back down, recite the tashahhud, and then perform the prostration of forgetfulness (sujood as-sahw) to complete their prayer. If they have already performed the prostration of the rak‘ah they stood up for, according to Imam Abu Hanifa and Imam Abu Yusuf, it is no longer possible to complete this prayer as an obligatory (fard) prayer. If the prayer being performed is two or four rak‘ahs, in this case, they should perform one more rak‘ah and complete the prayer. This prayer then becomes a voluntary (nafl) prayer. Afterwards, they must repeat the obligatory prayer. The reason for adding an extra rak‘ah when the prostration of the mistakenly performed rak‘ah has been done is that voluntary prayers are legislated to be performed in even numbers of rak‘ahs (Ibn Nujaym, al-Bahr, 2/112). If the prayer being performed is the maghrib prayer and the prostration of the rak‘ah they stood up for has not been done, as above, they should sit back down and complete the prayer with the prostration of forgetfulness. If the prostration of the rak‘ah has been performed, then the prayer will have been performed as four rak‘ahs. Since the prayer in this case becomes a voluntary prayer, the obligatory maghrib prayer must be performed again.

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu