Plato, Letters 321a

ἔλθοιτʼ ἄν. ταῦτα δὲ ἴσως γελοῖόν σοι φαίνεται εἶναι τὸ ἐμὲ λέγειν, διότι καὶ αὐτὸς οὐκ ἀγνοεῖς· ἐγὼ δὲ καὶ ἐν τοῖς θεάτροις ὁρῶ τοὺς ἀγωνιστὰς ὑπὸ τῶν παίδων παροξυνομένους, μήτι δὴ ὑπό γε τῶν φίλων, οὓς ἄν τις οἴηται μετὰ σπουδῆς κατʼ εὔνοιαν παρακελεύεσθαι. νῦν οὖν αὐτοί τε ἀγωνίζεσθε καὶ ἡμῖν εἴ του δεῖ ἐπιστέλλετε· τὰ δʼ ἐνθάδε παραπλησίως ἔχει καθάπερ καὶ ὑμῶν παρόντων. ἐπιστέλλετε

Probably it strikes you as ridiculous that I should say this, seeing that you yourself also know it quite well; but I notice how even in the theaters the players are spurred on by the plaudits of the children—not to speak of their own friends—whenever a player believes them to be genuine and well-meaning in their encouragement. So do you also play your parts now; and if you have need of anything send us word. Affairs with us are in much the same state as when you were here. Send us word also about what you have already done or happen to be doing now,