Salaah

Forgetting to sit for the last qa'dah in a four rakaat fardh salaah

Q: A person was performing a four rakaat fardh salaah. After performing the fourth rakaat, he did not sit for the qa'dah, but forgetfully stood up for the fifth rakaat. He then made ruku and sajdah in the fifth rakaat, sat for tashahhud and made salaam.

What is the status of his salaah? Is his fardh salaah valid or does he have to repeat his salaah?