Full Idea
He appears to have been the first to discover that Hesperus and Lucifer were the same star.
Gist of Idea
He was the first to discover the identity of the Morning and Evening Stars
Source
report of Parmenides (fragments/reports [c.474 BCE]) by Diogenes Laertius - Lives of Eminent Philosophers 09.Pa.3
Book Reference
Diogenes Laertius: 'Diogenes Laertius', ed/tr. Yonge,C.D. [Henry G. Bohn 1853], p.385
A Reaction
This is the famous example used by Frege to discuss reference and meaning.