Obligation for the Ma'mums to offer the fourth Rak`ah missed by the Imam

Q: The Imam (the one who leads congregational Prayer) of the Masjid (mosque) led us in a four-Rak‘ah Salah (Prayer consisting of four units). He sat down to recite the first Tashahhud (a recitation in the sitting position in the second/ last unit of Prayer) in the second Rak‘ah then rose up and offered one more Rak‘ah, and then sat down. The Ma’mums (those being led by an Imam in Prayer) said, “Subhan Allah [Glory be to Allah]”, to draw his attention to his mistake but he did not respond and said Taslim (salutation of peace ending the Prayer). Is the congregation obligated to complete the fourth Rak‘ah or follow the Imam then repeat the whole Salah?

A: The Imam and Ma’mums are both obliged to offer the fourth Rak‘ah to complete the Salah. The Imam should rise up to perform the fourth Rak‘ah by facing the Qiblah (direction faced for Prayer towards the Ka‘bah) while sitting, then standing up to perform the fourth Rak‘ah. The Ma’mums have to follow the Imam and not to pray on their own. The Imam should then offer the two prostrations of Sujud-ul-Sahw (Prostration of Forgetfulness). In case the Imam does not rise up after the Ma’mums have drawn his attention, they should rise up and complete their Salah and say Taslim and they are not required to do anything further.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.