Laudator Temporis Acti

Xenophon, Education of Cyrus 1.2.7 (tr. Walter Miller):

And they bring one another to trial also charged with an offence for which people hate one another most but go to law least, namely, that of ingratitude; and if they know that any one is able to return a favour and fails to do so, they punish him also severely. For they think that the ungrateful are likely to be most neglectful of their duty toward their gods, their parents, their country, and their friends; for it seems that shamelessness goes hand in hand with ingratitude; and it is that, we know, which leads the way to every moral wrong.

δικάζουσι δὲ καὶ ἐγκλήματος οὗ ἕνεκα ἄνθρωποι μισοῦσι μὲν ἀλλήλους μάλιστα, δικάζονται δὲ ἥκιστα, ἀχαριστίας, καὶ ὃν ἂν γνῶσι δυνάμενον μὲν χάριν ἀποδιδόναι, μὴ ἀποδιδόντα δέ, κολάζουσι καὶ τοῦτον ἰσχυρῶς. οἴονται γὰρ τοὺς ἀχαρίστους καὶ περὶ θεοὺς ἂν μάλιστα ἀμελῶς ἔχειν καὶ περὶ γονέας καὶ πατρίδα καὶ φίλους. ἕπεσθαι δὲ δοκεῖ μάλιστα τῇ ἀχαριστίᾳ ἡ ἀναισχυντία· καὶ γὰρ αὕτη μεγίστη δοκεῖ εἶναι ἐπὶ πάντα τὰ αἰσχρὰ ἡγεμών.

Hubert A. Holden in his commentary ad loc. quotes William Shakespeare, Twelfth Night 3.4.353-356 (here with John Dover Wilson's line numbering and text):

I hate ingratitude more in a man,
Than lying vainness, babbling drunkenness,
Or any taint of vice whose strong corruption
Inhabits our frail blood.

See also Jonathan Swift, Gulliver's Travels, Chapter VI ("Of the inhabitants of Lilliput..."):

Ingratitude is among them a capital crime, as we read it to have been in some other countries: for they reason thus, that whoever makes ill returns to his benefactor, must needs be a common enemy to the rest of mankind, from whom he hath received no obligation, and therefore such a man is not fit to live.