What is the ruling on the prayer of one who sat for the final Tashahhud but forgot the wording of the Tashahhud?

The final Tashahhud and sitting for it are two pillars of prayer without which the prayer is invalid. The proof for this is the Hadith of Ibn Mas'ud (may Allah be pleased with him): "We used to say, before the Tashahhud was obligated upon us: 'Peace be upon Allah from His servants, peace be upon Jibreel and Mika'eel, peace be upon so-and-so and so-and-so.'" These are not dropped due to forgetfulness. So, whoever forgets a pillar must perform it. If he has made the Taslim and the interval was not long, he must return, sit, perform the Tashahhud, then make the Taslim and perform the prostrations of forgetfulness (sujud as-sahw). But if the interval was long, he must repeat the entire prayer.