Laudator Temporis Acti

Greek Anthology 9.234 (Crinagoras, tr. D.L. Page):

How long, poor fool, fluttering on hopes as high as the chilly clouds, my soul, will you sketch dream upon dream of riches? Nothing comes to man's possession of its own accord. Pursue rather the Muses' gifts, and leave these dim phantoms of the mind to fools.

ἄχρι τεῦ, ἆ δείλαιε, κεναῖς ἐπὶ ἐλπίσι, θυμέ,
  πωτηθεὶς ψυχρῶν ἀσσοτάτω νεφέων
ἄλλοις ἄλλ' ἐπ' ὄνειρα διαγράψεις ἀφένοιο;
  κτητὸν γὰρ θνητοῖς οὐδὲ ἓν αὐτόματον.
Μουσέων ἀλλ' ἐπὶ δῶρα μετέρχεο, ταῦτα δ' ἀμυδρά
  εἴδωλα ψυχῆς ἠλεμάτοισι μέθες.

Gow and Page, commentary (The Garland of Philip) on line 4:

The point seems to be that it is no good merely dreaming of riches, hoping that they will fall into your lap; wealth will come only with effort—a kind of effort beyond the power of Crinagoras, who will be well advised to stop dreaming of becoming a millionaire and to make good use of the talents which the Muses have given him.