Full Idea
Everything which a thing is must be proved through a demonstration - except its essence. But existence is not the essence of anything; for the things that exist do not constitute a kind.
Gist of Idea
Properties must be proved, but not essence; but existents are not a kind, so existence isn't part of essence
Source
Aristotle (Posterior Analytics [c.327 BCE], 92b14)
Book Reference
Aristotle: 'Posterior Analytics (2nd ed)', ed/tr. Barnes,Jonathan [OUP 1993], p.55