Full Idea
It things are many, they can't be more or less than they are, so they must be finite, but also there must be endless things between each thing, so they must be infinite.
Gist of Idea
If there are many things they must have a finite number, but there must be endless things between them
Source
Zeno (Elea) (fragments/reports [c.450 BCE], B3), quoted by Simplicius - On Aristotle's 'Physics' 140.29
Book Reference
'Ancilla to the Pre-Socratic Philosophers', ed/tr. Freeman,Kathleen [Harvard 1957], p.47