Full Idea
Zeno first (in his book On Human Nature) said that the goal was to live in agreement with nature, which is to live according to virtue.
Gist of Idea
Zeno said live in agreement with nature, which accords with virtue
Source
report of Zeno (Citium) (fragments/reports [c.294 BCE]) by Diogenes Laertius - Lives of Eminent Philosophers 07.87
Book Reference
'The Stoics Reader', ed/tr. Inwood,B/Gerson,L.P. [Hackett 2008], p.113
A Reaction
The main idea seems to be Aristotelian - that the study of human nature reveals what our virtues are, and following them is what nature requires. Nature is taken to be profoundly rational.