Is it permissible to make up two rak'ahs and perform the prostration of forgetfulness (sujud al-sahw) after prostrating with the Imam in the position of bowing due to the recitation of an ayah of prostration (sajdat al-tilawah) in the 'Isha prayer, or must the prayer be repeated?

If a congregant misses the bowing (ruku') with the imam due to an excuse, such as being preoccupied, then he must perform it and then follow his imam, and his rak'ah (unit of prayer) is valid. However, if he does not perform it, then the rak'ah is void, and he must perform it after the imam concludes the prayer with taslim.