What is the ruling on someone who led the Dhuhr prayer and stood for an extra rak'ah in the last rak'ah, then returned to the Tashahhud and the Tasleem without performing the prostration of forgetfulness (sujud as-sahw) after being prompted with "SubhanAllah"?

The prescribed action for an Imam who stands for a fifth raka'ah is to perform the prostration of forgetfulness (sujud al-sahw). If he remembers before prostrating, he should return to the sitting position, recite the tashahhud, perform the prostration of forgetfulness, and then conclude the prayer with salutations. However, his omission of the prostration of forgetfulness does not invalidate the prayer, and the prayer of both the Imam and the congregants is valid.