Full Idea
Euclid's Postulate 2 says the geometer can 'produce a finite straight line continuously in a straight line'.
Gist of Idea
Postulate 2 says a line can be extended continuously
Source
report of Euclid (Elements of Geometry [c.290 BCE]) by Stewart Shapiro - Thinking About Mathematics 4.2
Book Reference
Shapiro,Stewart: 'Thinking About Mathematics' [OUP 2000], p.87
A Reaction
The point being that this takes infinity for granted, especially if you start counting how many points there are on the line. The Einstein idea that it might eventually come round and hit you on the back of the head would have charmed Euclid.