The prajnaparamita canon is quite ancient going back to 1-2 BCE. It's not clear if the original set of sutras were composed in Sanskrit or Gandhari first and subsequently translated to Sanskrit. (Source: Wikipedia)
However the heart (Hṛdaya) sutra, said to be part of the Prajñāpāramitā Sūtra canon, and most famous certainly, is in the view of modern historians and scholars almost certainly not part of the original Indian compositions. It was likely composed in Chinese and later rendered in Sanskrit.
This is a text that has caused a lot of confusion among students because it attempts to lay down the highest of realized truths using the inadequate vehicle of conventional language. Despite several attempts at translating it to be less obscure, it is a text that needs to be meditated upon, and not merely understood through the intellect reading it - (i.e. the language it is read in doesn't really matter as long as the translation is correct).
Here are some side by side Sanskrit-English translations:
... and one can find more of them using Google, but it is impossible to find the authentic original Sanskrit you're looking for (because there is none?).