An Imam (the one who leads congregational Prayer) led us in the Zhuhr (Noon) Prayer and in the second Rak`ah (unit of Prayer) before reciting the first part of Tashahhud (a recitation in the sitting position in the second/ last unit of Prayer), he forgetfully performed one Sujud (prostration) instead of two.
When he finished Salah (Prayer), a Ma'mum (a person being led by an Imam in Prayer) told him about his mistake, and asked him to repeat the Rak`ah and perform Sujud-ul-Sahw (Prostration of Forgetfulness). Is this correct? Is it better to just perform Sujud-ul-Sahw or to offer a Rak`ah then perform Sujud-ul-Sahw?
If the reality is as you mentioned, then the Rak`ah in which the Imam mistakenly missed Sujud is cancelled. Therefore, the third Rak`ah becomes the second and the fourth Rak`ah becomes the third. He, thus, should offer a fourth Rak`ah, then perform Sujud-ul-Sahw.