Is it obligatory for the Imam to perform a prostration (sujud) after reciting a verse of prostration in the Fajr prayer, then sitting and being alerted by the congregants, then completing the prayer without prostrating before or after the taslim (salutation)?

The Imam is required to perform the prostration of forgetfulness (sujud al-sahw) if he sat inadvertently during the prayer, because he added an extra sitting. The Shafi'is view it as a sunnah (recommended) and its place is before the salutation (taslim). The Hanbalis view it as obligatory and its place is before the salutation, and the prayer is not invalidated by inadvertently omitting it, even if a long interval passes.