eindesliebe Denys L. Page, in his commentary on Euripides' Medea 809-810, quotes Lessing:
Moral excellence in ancient Greece consisted no less in unremitting hatred of your foes than in unalterable love toward your friends.
Here is a selection of passages from Greek literature on the topic:
* Hesiod, Works and Days, 353-354: Be friends
with the friendly, and visit him who visits you. Give to one who gives, but
do not give to one who does not give.
* Theognis 869-872: May the great wide brazen sky fall upon me -- that dread of earthborn men -- if I aid not such as love me, and become not a pain and great grief unto such as hate.
* Archilochus, fragment 23: I know how to love the one who loves me and hate the enemy.
* Archilochus, fragment 126: One important thing I know, how to repay with terrible evils the one who mistreats me.
* Solon, fragment 13 West (a prayer to the Muses, lines 5-6, tr. J.M. Edmonds): Make me, I pray you, sweet to my friends and sour to my foes, to these a man reverend to behold, to those a man terrible.
* Pindar, Pythian Odes 2.83-85: Let me befriend a friend, but against an enemy, I shall, as his enemy, run him down as a wolf does, stalking now here, now there, on twisting paths.
* Aeschylus, Libation Bearers 122-123: ELECTRA. And is this a righteous thing for me to ask of Heaven? CHORUS. Righteous? How not? To requite an enemy evil for evil!
* Sophocles, Antigone 641-644: This is why men pray that they may beget and keep in their houses obedient offspring, so that they may requite the enemy with evil and honour the friend as they honour their father.
* Euripides, Medea 807-810: Let no one think me weak, contemptible, untroublesome. No, quite the opposite, hurtful to foes, to friends kindly. Such persons live a life of greatest glory.
* Euripides, Heracles 585-586: It is in your nature, my son, to be loving to your friends and to hate your enemies.
* Euripides, Ion 1046-1047: But when a man wants to harm an enemy, no law stands in the way.
* Lysias 9.20: I considered it ordained that one should harm one's enemies and serve one's friends.
* Xenophon, Memorabilia 2.6.35: A man's virtue consists in outdoing his friends in kindness and his enemies in mischief.
* Xenophon, Anabasis 1.9.11: It was manifest also that whenever a man conferred any benefit upon Cyrus or did him any harm, he always strove to outdo him; in fact, some people used to report it as a prayer of his that he might live long enough to outdo both those who benefited and those who injured him, returning like for like.
* Xenophon, Hiero 2.2: You are rich in power to harm enemies and reward friends.
* Plato, Republic 1.7.332d: Justice is the art which gives good to friends and evil to enemies.