How should we perform Sujud-ul-Sahw

Q: A man led the people in Maghrib (Sunset) Prayer, which was offered completely. When he sat for saying the last Tashahhud (testification recited in the sitting position in the second/last unit of Prayer) and then said the first Taslim (salutation of peace ending the Prayer), one of the followers said, "Subhana Allah" [hinting that the Imam forgot something]. (Part No. 7; Page No. 130) Therefore, the Imam (the one who leads congregational Prayer) stood and offered an additional Rak`ah (unit of Prayer). We followed him and offered it. Then he sat down to say Tashahhud and before Taslim, he offered Sujud-ul-Sahw (Prostration of Forgetfulness). After finishing, one of the followers said, "One who is not sure that the Imam missed [any part of prayer] should keep silent." The Imam added, "If anyone [of the followers] is certain that their prayer is complete, should keep sitting when the Imam stands to perform the [additional] Rak`ah, during which they should remember Allah until the Imam sits again to say the Tashahhud and say Taslim. Then that follower says Taslim after the Imam." At that moment I said, "But, the follower should follow the Imam in everything; the Messenger of Allah (peace be upon him) said, 'The Imam is appointed only to be followed; so when he recites Takbir, you should also recite it...'"

A: Anyone who stood with the Imam in the mentioned prayer and offered an extra Rak`ah while knowing that it was extra and knew its legal ruling, their Salah (prayer) is Batil (null and void). This is because they intentionally prayed Maghrib Prayer four Rak`ahs. But anyone who followed the Imam unknowing that they increased or thinking that they should pray another Rak`ah and followed the Imam, their Salah is valid. Allah (Exalted be He) said, Our Lord! Punish us not if we forget or fall into error It was confirmed that the Messenger of Allah (peace be upon him) said, "Verily, Allah (Praised and Exalted be He) [in reference to the above verse] said, 'I indeed did it'." It was also narrated the he (peace be upon him) said, "Allah has forgiven my Ummah (nation) what they did by mistake, forgetfulness and what they did under coercion." (Part No. 7; Page No. 131) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.