What is the ruling on the prayer of one who forgot the middle Tashahhud and did not perform the prostration of forgetfulness (sujud al-sahw)? Is his prayer valid, and if not, is a make-up (qada') obligatory? And what about the prayer of the congregant (ma'mum)?

If the congregant alerts the Imam after he has stood up from the first tashahhud, and the Imam continues his prayer, the prayer remains valid, but the Imam must perform the prostration of forgetfulness (sujood as-sahw) for omitting the tashahhud. If he does not prostrate, the jurists differ on the validity of the prayer, and it is more prudent to repeat it. The congregant must perform the prostration of forgetfulness even if the Imam omits it.