Phaedrus 4.20 (tr. Ben Edwin Perry):
He who brings aid to the wicked afterwards suffers for it.
A man picked up a venomous serpent benumbed by the cold
and warmed it in his bosom, showing pity to his own cost;
for when the serpent revived he immediately killed the man.
When another serpent asked him why he did this, 5
he replied: "To teach men not to be good to those who are no good."
Qui fert malis auxilium, post tempus dolet.
Gelu rigentem quidam colubram sustulit
sinuque fovit, contra se ipse misericors;
namque, ut refecta est, necuit hominem protinus.
hanc alia cum rogaret causam facinoris, 5
respondit "Ne quis discat prodesse improbis."
Babrius 143 (tr. Ben Edwin Perry):
A farmer picked up a viper that was almost dead from the cold, and warmed it. But the viper, after stretching himself out, clung to the man's hand and bit him incurably, thus killing (the very one who wanted to save him). Dying, the man uttered these words, worthy to be remembered: "I suffer what I deserve, for showing pity to the wicked."
Ἔχιν γεωργὸς ἐκπνέοντ᾿ ὑπὸ ψύχους
λαβὼν ἔθαλπεν· ἀλλ᾿ ἐκεῖνος ἡπλώθη
τῇ χειρὶ προσφύς, καὶ δακὼν ἀνιήτως
ἔκτεινεν [αὐτὸν τὸν θέλοντ᾿ ἀνστῆσαι.]†
θνῄσκων δὲ μῦθον εἶπεν ἄξιον μνήμης· 5
"δίκαια πάσχω τὸν πονηρὸν οἰκτείρας."
Aesop 82 Chambry (my translation):
A farmer in winter time found a serpent stiff with cold. After pitying the serpent and picking it up, he placed it beneath his garment's fold. The serpent, warmed up and reverting to its nature, struck and killed its benefactor. As he was dying, the man said, "I'm getting what I deserve for having taken pity on a wicked creature."
Γεωργός τις χειμῶνος ὥρᾳ ὄφιν εὑρὼν ὑπὸ κρύους πεπηγότα, τοῦτον ἐλεήσας καὶ λαβὼν ὑπὸ κόλπον ἔθετο. Θερμανθεὶς δὲ ἐκεῖνος καὶ ἀναλαβὼν τὴν ἰδίαν φύσιν ἔπληξε τὸν εὐεργέτην καὶ ἀνεῖλε· θνῄσκων δὲ ἔλεγε· Δίκαια πάσχω, τὸν πονηρὸν οἰκτείρας.
This is what happens to those who obey Matthew 5.44:
Do good to them that hate you.