Erasmus, Adagia 1.1.33:

“Socrates, in Plato’s Axiochus, says to the sophist Prodicus that this verse of the comic poet Epicharmus was always on his tongue:

Ἡ δὲ χεὶρ τὴν χεῖρα κνίζει, δός τι καὶ λάβοις τι,

that is,

‘One hand rubs the other – give something and receive something.’

Obviously this was spoken upbraiding the acquisitiveness of a man who would teach no one for free and by whom, as he often affirmed, he had learned what he was going to say, and that not free, but only after paying a fee. The saying is variously applied to a Sicilian and to a cunning poet; for thus Cicero names him. He does however warn that no person can be found who would do a favor for another from whom he did not expect that he would receive a favor in return – rather, kindness is elicited by kindness, and favors…

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