Laudator Temporis Acti

Plutarch, On the Fortune or the Virtue of Alexander 6 = Moralia 329 a-b (tr. Frank Cole Babbitt):

Moreover, the much-admired Republic of Zeno, the founder of the Stoic sect, may be summed up in this one main principle: that all the inhabitants of this world of ours should not live differentiated by their respective rules of justice into separate cities and communities, but that we should consider all men to be of one community and one polity, and that we should have a common life and an order common to us all, even as a herd that feeds together and shares the pasturage of a common field. This Zeno wrote, giving shape to a dream or, as it were, shadowy picture of a well-ordered and philosophic commonwealth; but it was Alexander who gave effect to the idea.

καὶ μὴν ἡ πολὺ θαυμαζομένη Πολιτεία τοῦ τὴν Στωικῶν αἵρεσιν καταβαλομένου Ζήνωνος εἰς ἓν τοῦτο συντείνει κεφάλαιον, ἵνα μὴ κατὰ πόλεις μηδὲ κατὰ δήμους οἰκῶμεν ἰδίοις ἕκαστοι διωρισμένοι δικαίοις, ἀλλὰ πάντας ἀνθρώπους ἡγώμεθα δημότας καὶ πολίτας, εἷς δὲ βίος ᾖ καὶ κόσμος, ὥσπερ ἀγέλης συννόμου νομῷ κοινῷ συντρεφομένης. τοῦτο Ζήνων μὲν ἔγραψεν ὥσπερ ὄναρ ἢ εἴδωλον εὐνομίας φιλοσόφου καὶ πολιτείας ἀνατυπωσάμενος, Ἀλέξανδρος δὲ τῷ λόγῳ τὸ ἔργον παρέσχεν.

Slightly more literally — "as of a herd," etc.