What is the ruling on someone who forgot the first Tashahhud, and then performed a complete raka'ah after the final Tashahhud, and prostrated for forgetfulness (sujud al-sahw) before the salam?

The first Tashahhud (definition) is obligatory, and whoever misses it forgetfully should perform the prostration of forgetfulness (sujūd as-sahw) before the salām. However, adding an extra unit of prayer (rak'ah) to compensate for the forgotten Tashahhud is a mistake. If this addition also occurred forgetfully, then two prostrations of forgetfulness before the salām for both instances are sufficient. There is no need to repeat the prayer for adding a rak'ah out of ignorance.