What should a worshiper do if they forget in a two-rak'ah prayer, such as Fajr, its Sunnah, or Qiyam al-Layl, and stand up after the second rak'ah?

If someone, out of forgetfulness, performs an extra rak'ah in a four-unit prayer, or the Maghrib prayer, or a supererogatory prayer, they must return (to the correct number of rak'ahs) as soon as they remember, because these prayers have a fixed number of rak'ahs. They should perform the prostration of forgetfulness (sujūd al-sahw) after the salutation (salām), provided they are not a ma'mūm (follower in congregational prayer) and have not yet placed their hands and knees on the ground. The Malikis hold that in supererogatory prayers (other than the Raghibat al-Fajr – the two rak'ahs before Fajr), if one has already bowed for the third rak'ah, they should complete it as four rak'ahs and perform the prostration of forgetfulness before the salutation. However, concerning Raghibat al-Fajr, they must return (to the correct number) even if they have already bowed. If they do not return, knowing that they have added an extra rak'ah, their prayer becomes invalid, unless they are excused due to ignorance.