Full Idea
Stoics say the beautiful is the only good. Good is an equivalent term to the beautiful; since a thing is good, it is beautiful; and it is beautiful, therefore it is good.
Gist of Idea
Stoics say that beauty and goodness are equivalent and linked
Source
report of Chrysippus (fragments/reports [c.240 BCE]) by Diogenes Laertius - Lives of Eminent Philosophers 07.1.59
Book Reference
Diogenes Laertius: 'Diogenes Laertius', ed/tr. Yonge,C.D. [Henry G. Bohn 1853], p.296