Erasmus, Adagia 33:

Socrates in the Axiochus of Plato, says to the sophist Prodicus that this little verse of the comic Epicharmus was always in his mouth: Ἡ δὲ χεὶρ τὴν χεῖρα κνίζει, δός τι καὶ λάβοις τι, that is, one hand wipes the other, give something and get something, obviously reproaching in a humorous way the greed of a man who taught no one for free, and from whom he affirmed that he himself had learned what he was about to speak not for free, but by paying a fee. This idea was worthy then of a Sicilian, then of a ‘cunning poet.’ For thus Cicero labels him. It does, however, advise us that no one can be found who would wish to do someone a service without wishing that the other would repay the favor in turn: rather, duty is called forth by duty, favors called forth by favors. The same adage is expressed this way: Χεὶρ χεῖρα νίπτει, that is, one hand washes the other. Either metaphor has the same sense, for it is a kind of shared benefit when one hand either wipes or washes the other. There is a distich of this sort which was fashionable among the Greeks: Ἀνὴρ γὰρ ἄνδρα καὶ πόλις σῴζει πόλιν.  Χεὶρ χεῖρα νίπτει, δάκτυλος τε δάκτυλον, that is, one man saves another, one citiy saves the other; the hand washes the hand and the finger cleans the finger. Seneca uses this phrase in that ridiculous little book about the death of Claudius.

MANVS MANVM FRICAT

              Socrates in Axiocho Platonis ait Prodico sophistae hunc Epicharmi comici versiculum semper in ore fuisse:  Ἡ δὲ χεὶρ τὴν χεῖρα κνίζει, δός τι καὶ λάβοις τι,  id est Affricat manum manus, da quiddam et aliquid accipe, videlicet hominis quaestum facete taxans, qui neminem gratis doceret et a quo se quoque quae tum dicturus esset, didicisse affirmabat, at ne id quidem gratuito, imo numerata mercede. Sententia digna tum homine Siculo tum ‘vafro poeta’; sic enim illum appellat Cicero. Monet autem neminem ferme mortalium inueniri, qui velit in quempiam beneficium collocare, a quo non speret aliquid emolumenti vicissim ad se rediturum, sed officium inuitari officio, beneficium beneficio prouocari. Idem adagium effertur et hoc pacto: Χεὶρ χεῖρα νίπτει, id est Manum manus lauat. Idem pollet vtraque metaphora. Nam mutua commoditas est, quoties vel fricat vel abluit manus manum. Circunfertur inter Graecanicas sententias huiusmodi distichon: Ἀνὴρ γὰρ ἄνδρα καὶ πόλις σῴζει πόλιν.

Erasmus, Adagia 30:

The same thought is put forth by others: Ῥεχθὲν δε τε νήπιος ἔγνω, that is, ‘The fool understands the matter once it’s done.’ It has, however, been taken from Homer, who has taken up this thought in many places, as in Μήτε ἀντίος ἵστασ᾽ ἐμοῖο,  Πρίν τι κακὸν παθέειν· ῥεχθὲν δέ τε νήπιος ἔγνω, that is, Beware of coming up against me, before you take up some harm; for even the stupid person understands the deed once it’s done. Euripides has alluded to this in his Bacchae: Κακοῦ γὰρ ἐγγὺς ὢν ἐμάνθανεν, that is, For he learned, being close to the misfortune; this was said of Pentheus, who learned too late and not without danger to himself to revere Bacchus. Not entirely dissimilar to this is that senarius renowned among the Greek sententiae: Ἡ δὲ μετάνοια γίγνετ᾽ ἀνθρώποις κρίσης, that is, Then people judge, when they already have regret. This expression of Vergil has the same bearing: Having been warned, learn justice and not to spurn the gods. Similarly, the thought of Demosthenes: I do not purchase regret at such a price. And so, in the most elegant way, Fabius (in Livy) calls the outcome the teacher of the fool, saying, Nor would the outcome, which is the teacher of the fool, teach this. Pliny, in his Panegyric which he spoke to Trajan, calls prudence of this sort fruitless and wretched, saying Terror, and fear, and that wretched prudence made out of dangers warned us to turn our eyes, our ears, our minds away from the republic (though there was, however, no republic at all).

FACTVM STVLTVS COGNOSCIT

Idem aliter effertur ab aliis: Ῥεχθὲν δε τε νήπιος ἔγνω, id est• Rem peractam stultus intellexit•. Sumptum est autem ex Homero, qui pluribus locis hanc vsurpauit sententiam. Vt in Iliados Μήτε ἀντίος ἵστασ᾽ ἐμοῖο,  Πρίν τι κακὸν παθέειν· ῥεχθὲν δέ τε νήπιος ἔγνω,  id est  Mihi obuius ire caueto, prius quam Noxae aliquid capias; nam factum nouit et excors. Huc allusit Euripides in Bacchis: Κακοῦ γὰρ ἐγγὺς ὢν ἐμάνθανεν,  id est Nam didicit affinis malo, de Pentheo, qui sero nec nisi sua pernicie doctus coepit reuereri Bacchum. Neque huic diuersum est, quod admonet senarius ille inter Graecanicas sententias celebris:  Ἡ δὲ μετάνοια γίγνετ᾽ ἀνθρώποις κρίσης,  id est  Tum iudicant homines, vbi iam poenitet. Eodem pertinet Vergilianum illud: Discite iustitiam moniti et non temnere diuos. Item illud Demosthenicum: Non emo tanti poenitere. Vnde perquam eleganter Fabius apud Titum Liuium euentum stultorum magistrum appellat Nec euentus doceat hoc, inquiens, qui stultorum magister est, sed ratio. Plinius in Panegyrico, quem Traiano dixit, huiusmodi seram et infrugiferam prudentiam miseram vocat. Terror, inquit, et metus et misera illa ex periculis facta prudentia monebat, vt a republica (erat autem omnino nulla respublica) oculos, aures, animos auerteremus.