Hesiod. Works and Days. (Lines 274-285).
But you, Perses, lay up these things within you heart and listen now to right, ceasing altogether to think of violence. For the son of Cronos has ordained this law for men, that fishes and beasts and winged fowls should devour one another, for right is not in them; but to mankind he gave right which proves far the best. For whoever knows the right and is ready to speak it, far-seeing Zeus gives him prosperity; but whoever deliberately lies in his witness and forswears himself, and so hurts Justice and sins beyond repair, that man's generation is left obscure thereafter. But the generation of the man who swears truly is better thenceforward.