Is the prostration of forgetfulness (sujud al-sahw) obligatory if the first tashahhud was not recited upon entering the second rak'ah of the Isha prayer?

If a congregant accidentally omits the first tashahhud, prostration of forgetfulness (sujūd as-sahw) is not prescribed for him, because the Imam bears it on his behalf as long as he is following him. Therefore, the congregant does not perform prostration of forgetfulness unless his Imam forgets, in which case he prostrates with him.