What is the ruling on the prayer of the imam and the congregants if the imam stood for a fifth rak'ah by mistake, then returned to the final tashahhud and did not perform the prostration of forgetfulness (sujud al-sahw)?

The ruling for this Imam is to perform two prostrations after the Taslim, and the congregants should prostrate with him. However, their omission of the prostrations of forgetfulness does not invalidate the prayer, and their prayer, all together, is valid.