Full Idea
The First is no member of existence, but can be the source of all.
Gist of Idea
The One does not exist, but is the source of all existence
Source
Plotinus (The Enneads [c.245], 5.1.07)
Book Reference
Plotinus: 'The Enneads', ed/tr. Mackenna,Stephen [Penguin 1991], p.355
A Reaction
The First is the One, and this explicitly denies that it has Being. This answers the self-predication problem of Forms. Plato thought the Form of the Beautiful was beautiful, but it can't be (because of the regress). The source of existence can't exist.