What is the ruling on someone who prayed Maghrib and forgot the first tashahhud, then was reminded by a congregant, so he sat for the first tashahhud, and then completed his prayer?

If a worshipper forgets the first [middle] tashahhud and remembers it or is reminded of it before standing upright, he should sit for the tashahhud and does not need to perform the prostrations of forgetfulness (sujood al-sahw). However, if he remembers it or is reminded of it after having stood upright completely, he should not return [to sit] but should proceed with his prayer, and he is required to perform two prostrations of forgetfulness before the tasleem. If he stands upright completely and then returns intentionally, knowing that it is forbidden, his prayer becomes void.