Iranian languages
Old Persian is the Old Iranian dialect as it was spoken in south-western Iran by the inhabitants of Parsa, who also gave their name to their region and language. Genuine Old Persian is best attested in one of the three languages of the Behistun inscription, composed circa 520 BC, and which is the last inscription (and only inscription of significant length) in which Old Persian is still grammatically correct. Later inscriptions are comparatively brief, and typically simply copies of words and phrases from earlier ones, often with grammatical errors, which suggests that by the 4th century BC the transition from Old Persian to Middle Persian was already far advanced, but efforts were still being made to retain an "old" quality for official proclamations.

Q: In which historical record does Old Persian appear in exemplary form?
the Behistun inscription