Is the prostration of forgetfulness (sujud al-sahw) legislated if the Imam sits during the third rak'ah, believing it to be the fourth, then stands up immediately after being alerted by the congregants and performs the fourth (rak'ah)?

If a worshipper sits during the third rak'ah by mistake, it is prescribed for him to perform the prostrations of forgetfulness (sujud al-sahw) due to the generality of the Prophet's (peace be upon him) saying: "For every forgetfulness, there are two prostrations." However, if his sitting was brief and he did not prostrate, there is no harm upon him, because it is a minor action similar to the jalsat al-istirahah (sitting for rest).