Xenophon, Memorabilia 3.12

“For all of the uses of the body it makes a big difference to keep it in as good a condition as possible. Even for thinking, in which the use of the body seems least important, who does not know that many things fail in its practice because the body is not healthy? Forgetfulness, depression, ill temper and madness often strike the mind so badly because of bodily afflictions that it drives out understanding.

There is great stability for those who have strong bodies and there is, at least, no danger from suffering something like this because of physical affliction. No, it is likely that the useful help will develop as the opposite to those things that happen from affliction. And, indeed, what wouldn’t someone who has some sense try to forestall the opposite to those things I have mentioned?”

πάσαις δὲ ταῖς τοῦ σώματος χρείαις πολὺ διαφέρει ὡς βέλτιστα τὸ σῶμα ἔχειν· 6ἐπεὶ καὶ ἐν ᾧ δοκεῖς ἐλαχίστην σώματος χρείαν εἶναι, ἐν τῷ διανοεῖσθαι, τίς οὐκ οἶδεν, ὅτι καὶ ἐν τούτῳ πολλοὶ μεγάλα σφάλλονται διὰ τὸ μὴ ὑγιαίνειν τὸ σῶμα; καὶ λήθη δὲ καὶ ἀθυμία καὶ δυσκολία καὶ μανία πολλάκις πολλοῖς διὰ τὴν τοῦ σώματος καχεξίαν εἰς τὴν διάνοιαν ἐμπίπτουσιν οὕτως, ὥστε καὶ τὰς ἐπιστήμας ἐκβάλλειν. 7τοῖς δὲ τὰ σώματα εὖ ἔχουσι πολλὴ ἀσφάλεια καὶ οὐδεὶς κίνδυνος διά γε τὴν τοῦ σώματος καχεξίαν τοιοῦτόν τι παθεῖν, εἰκὸς δὲ μᾶλλον πρὸς τὰ ἐναντία τῶν διὰ τὴν καχεξίαν γιγνομένων τὴν εὐεξίαν χρήσιμον εἶναι. καίτοι τῶν γε τοῖς εἰρημένοις ἐναντίων ἕνεκα τί οὐκ ἄν τις νοῦν ἔχων ὑπομείνειεν;

Xenophon elaborates on some of this earlier

Xenophon, Memorabilia 3.5

“Certainly it is necessary—since the city does not provide public expenses for war—not to overlook it privately, nor otherwise to care for yourself less. Know well that you be no worse off in any other struggle or action because you have put your body in better shape. For the body is useful in everything people do. In all functions of the body it makes a big difference that the body is as healthy as possible. Even in something you might think the body is of little use—thinking—who doesn’t know that great errors come from having a sick body?

Forgetfulness, loss of spirit, ill-temper and madness often impinge upon perception because of the weakness of the body so badly that all knowledge is expelled. But for those who are healthy in body it is a great protection and they suffer no suffer no such risk of suffering this kind of thing because of the weakness of their body. It is probably that for those who have a healthy condition they will have the opposite experience. And, certainly, won’t anyone with some sense endure anything for the opposite of these things that have been mentioned?”

…

Anyway, is it not shameful to grow old because of carelessness before seeing how beautiful and strong a person you might be thanks to your body? It is not possible to witness this for someone who doesn’t make an effort. For it is not willing to develop on its own.”

Οὔτοι χρὴ ὅτι ἡ πόλις οὐκ ἀσκεῖ δημοσίᾳ τὰ πρὸς τὸν πόλεμον, διὰ τοῦτο καὶ ἰδίᾳ ἀμελεῖν, ἀλλὰ μηδὲν ἧττον ἐπιμελεῖσθαι. εὖ γὰρ ἴσθι, ὅτι οὐδὲ ἐν ἄλλῳ οὐδενὶ ἀγῶνι οὐδὲ ἐν πράξει οὐδεμιᾷ μεῖον ἕξεις διὰ τὸ βέλτιον τὸ σῶμα παρεσκευάσθαι· πρὸς πάντα γάρ, ὅσα πράττουσιν ἄνθρωποι, χρήσιμον τὸ σῶμά ἐστιν· ἐν πάσαις δὲ ταῖς τοῦ σώματος χρείαις πολὺ διαφέρει ὡς βέλτιστα τὸ σῶμα ἔχειν· ἐπεὶ καὶ ἐν ᾧ δοκεῖς ἐλαχίστην σώματος χρείαν εἶναι, ἐν τῷ διανοεῖσθαι, τίς οὐκ οἶδεν, ὅτι καὶ ἐν τούτῳ πολλοὶ μεγάλα σφάλλονται διὰ τὸ μὴ ὑγιαίνειν τὸ σῶμα; καὶ λήθη δὲ καὶ ἀθυμία καὶ δυσκολία καὶ μανία πολλάκις πολλοῖς διὰ τὴν τοῦ σώματος καχεξίαν εἰς τὴν διάνοιαν ἐμπίπτουσιν οὕτως, ὥστε καὶ τὰς ἐπιστήμας ἐκβάλλειν. τοῖς δὲ τὰ σώματα εὖ ἔχουσι πολλὴ ἀσφάλεια καὶ οὐδεὶς κίνδυνος διά γε τὴν τοῦ σώματος καχεξίαν τοιοῦτόν τι παθεῖν, εἰκὸς δὲ μᾶλλον πρὸς τὰ ἐναντία τῶν διὰ τὴν καχεξίαν γιγνομένων τὴν εὐεξίαν χρήσιμον εἶναι. καίτοι τῶν γε τοῖς εἰρημένοις ἐναντίων ἕνεκα τί οὐκ ἄν τις νοῦν ἔχων ὑπομείνειεν;

Αἰσχρὸν δὲ καὶ τὸ διὰ τὴν ἀμέλειαν γηρᾶναι, πρὶν ἰδεῖν ἑαυτὸν ποῖος ἂν κάλλιστος καὶ κράτιστος τῷ σώματι γένοιτο. ταῦτα δὲ οὐκ ἔστιν ἰδεῖν ἀμελοῦντα· οὐ γὰρ ἐθέλει αὐτόματα γίγνεσθαι.

Diogenes Laertius, 1.37.2

“When someone asked who is lucky, [Thales said] “whoever has a healthy body, a sophisticated mind, and teachable nature.”

τίς εὐδαίμων, “ὁ τὸ μὲν σῶμα ὑγιής, τὴν δὲ ψυχὴν εὔπορος, τὴν δὲ φύσιν εὐπαίδευτος.”

Juvenal, Satire 10.356

“We must beg for a healthy mind in a healthy body”

orandum est ut sit mens sana in corpore sano