Μία χελιδὼν ἔαρ οὐ ποιεῖ, οὐδὲ μία ἡμέρα.

“One swallow does not make a summer, nor does one fine day.”

A single good sign or instance is not enough to prove lasting happiness or virtue.

— Aristotle, Greece

Source: Aristotle, Nicomachean Ethics, Book I (circa 4th century BC)