Enchiridion · §32

ὅταν μαντείᾳ χρήσασθαι δέῃ, μέμνησο ὅτι ὁποῖά τινα ἀποβήσεται οὐκ οἶδας, ἀλλ᾽ ἥκεις ἵνα μάθῃς αὐτὰ παρὰ τοῦ μάντεως. ὁποῖα δέ ἐστιν ἤδη εἰδὼς ἥκεις, εἴπερ φιλόσοφος εἶ. εἰ γάρ τι τῶν οὐκ ἐφ᾽ ἡμῖν ἐστιν, ἀνάγκη πᾶσα μήτε ἀγαθὸν αὐτὸ εἶναι μήτε κακόν.μὴ φέρε οὖν πρὸς τὸν μάντιν ὄρεξιν ἢ ἔκκλισιν, ἀλλ᾽ ἐγνωκὼς ὅτι πᾶν τὸ ἀποβησόμενον ἀδιάφορον καὶ πρὸς σὲ οὐδέν.

hotan manteia chrēsasthai deē, memnēso hoti hopoia tina apobēsetai ouk oidas, all' hēkeis hina mathēs auta para tou manteōs. hopoia de estin ēdē eidōs hēkeis, eiper philosophos ei. mē phere oun pros ton mantin orexin ē ekklisin, all' egnōkōs hoti pan to apobēsomenon adiaphoron kai pros se ouden.

When you consult a diviner, remember that you do not know how the outcome will turn out — that is why you are going to find out. But what kind of outcome it is you already know, if you are a philosopher: it will be something outside your control, and therefore neither good nor bad in the fundamental sense. Do not bring desire or aversion when you consult the oracle — go having already decided that whatever comes is simply indifferent, and that it is up to you to use it well.

— Enchiridion 32

Source: Greek (Schenkl/Oldfather critical edition tradition) + Long 1877 English translation