Laudator Temporis Acti

Phocylides, fragment 7 (tr. J.M. Edmonds):

If thou desirest riches, see that thou hast a fertile farm;
for a farm, they say, is a horn of Amalthea.

χρηίζων πλούτου μελέτην ἔχε πίονος ἀγροῦ·
ἀγρὸν γάρ τε λέγουσιν Ἀμαλθείης κέρας εἶναι.

William Smith, Classical Dictionary, s.v. Amalthea:

Amalthea was a nymph, daughter of Oceanus, Helios, Haemonius, or of the Cretan king Melisseus, who fed Zeus with the milk of a goat. When this goat broke off one of her horns, Amalthea filled it with fresh herbs and gave it to Zeus, who placed it among the stars. According to other accounts Zeus himself broke off one of the horns of the goat Amalthea, and gave it to the daughters of Melisseus, and endowed it with the wonderful power of becoming filled with whatever the possessor might wish. This is the story about the origin of the celebrated horn of Amalthea, commonly called the horn of plenty or cornucopia, which was used in later times as the symbol of plenty in general.