An Imam (leader of congregational Prayer) led people in the Maghrib (Sunset) Prayer. He performed Ruku` (bowing), the first Sajdah (Prostration) and forgot to perform the second Sajdah. After he finished Salah (Prayer) and before Taslim (salutation of peace ending the Prayer), he performed Sujud-ul-Sahw (Prostration of Forgetfulness).
Is his Salah valid? What should he do so that his Salah becomes valid? May Allah reward you for what you do for Islam and the Muslims!
The second Sajdah is a Rukn (integral pillar) of Salah. A Rukn must be done; it cannot be compensated for by Sujud-ul-Sahw. As he did not perform it, and a long time has passed, you should repeat this Salah, as it was invalidated by missing the second Sajdah of the first Rak`ah. If the Imam had remembered or had been alerted after Taslim and before a long time passes, he should have offered another Rak`ah instead of the one in which he forgot a Sajdah, and he should also have performed Sujud-ul-Sahw. It is better to perform this Sajdah after Taslim. If he had remembered or had been alerted before Taslim, he should have offered another Rak`ah instead of the one in which he forgot a Sajdah, and then performed Sujud-ul-Sahw.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.