Missing one Rak`ah and only realizing after Taslim

A: If they had said the Taslim for the Zhuhr Prayer, for example, with a Rak`ah missing from the Salah, it is obligatory on them to offer it and also to perform Sujud-ul-Sahw (Prostration of Forgetfulness), if the time (between the Taslim and offering the missing Rak`ah) is not too long. However, if there is a long break before they offer the missing Rak`ah, they will be obliged to repeat the whole Salah.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.