Full Idea
The essence (to ti en einai), whose account (logos) is a definition, is also said to be the substance (ousia) of the particular. So there are two accounts of 'ousia' - as ultimate subject (hupokeimenon), never predicated of others, or as a this (tode ti).
Gist of Idea
Essence (fixed by definition) is also 'ousia', so 'ousia' is both ultimate subject, and a this-thing
Source
Aristotle (Metaphysics [c.324 BCE], 1017a22-)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.127
A Reaction
This slightly muddling assertion seems to be a report of how people use 'ousia', rather than Aristotle's theory. Attempts to translate this idea into English make fascinating reading! Hang on to the Greek, or you'll never get the hang of it!