Full Idea
In a Being one per se a real union is required consisting not in the situation or motion of parts, as in a chain or a house, but in a unique individual principle and subject of attributes and operations, in us a soul and in a body a substantial form.
Gist of Idea
A true being must (unlike a chain) have united parts, with a substantial form as its subject
Source
Gottfried Leibniz (De Mundo Praesenti [1686], A6.4.1506), quoted by Daniel Garber - Leibniz:Body,Substance,Monad 7
Book Reference
Garber,Daniel: 'Leibniz: Body, Substance, Monad' [OUP 2009], p.293
A Reaction
Leibniz is said not to be an essentialist, by making all properties essential, but he is certainly committed to substance, and it sounds like essence here (or one view of essence), when it makes identity possible. This idea is pure Aristotle.