Not sitting between the two Sujud and not performing the second Sujud
By missing the second Sujud
and the sitting between the two Sujud, this Imam omitted two Rukns (Pillars) of Salah (Prayer). If someone omits one or more Rukn of Salah, and does not remember until after they start the recitation in the next Rak‘ah, the current Rak‘ah will replace the previous Rak‘ah which was Batil (null and void), because of the lacking Rukn. They should, therefore, complete their Salah on that basis and perform Sujud-ul-Sahw. It is not permissible to return to perform Sujud after having already started the recitation for the following Rak‘ah. Therefore, what the Imam did of going back to make up for what he missed was not permissible, as he based it on an invalid Rak‘ah and also the time intervening between the two actions was long. Consequently, all the congregation have to repeat the whole Salah.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.