Enchiridion · §16

ὅταν κλαίοντά τινα ἴδῃς ἐν πένθει ἢ τέκνου ἀποδημήσαντος ἢ ἀπολωλότος τῶν αὐτοῦ, πρόσεχε μὴ σε ἡ φαντασία συναρπάσῃ ὡς ἐν κακοῖς αὐτοῦ ὄντος τοῦ ἐκτός· ἀλλ᾽ εὐθὺς ἔστω πρόχειρον ὅτι "τοῦτον θλίβει οὐ τὸ γεγονός, ἀλλὰ τὸ δόγμα τὸ περὶ τούτου". μέχρι μέντοι λόγου μὴ ὄκνει αὐτῷ συμπεριφέρεσθαι καὶ ἐὰν οὕτω τύχῃ συνεπιστενάξαι· πρόσεχε μέντοι μὴ καὶ ἔσωθεν στενάξῃς.

hotan klaionta tina idēs en penthei ē teknou apodēmēsantos ē apolōlotos tōn autou, proseche mē se hē phantasia sunarpasē hōs en kakois autou ontos tou ektos; all' euthus estō procheiron hoti 'touton thlibei ou to gegonos, alla to dogma to peri toutou'.

When you see someone weeping with grief — whether because a child has gone away or because they have lost their property — be careful that the impression does not sweep you into believing that the external event itself is a bad thing. Instead, have ready immediately the thought: 'What distresses this person is not what has happened, but their judgment about what has happened.' In your words, do not hesitate to show sympathy and even to groan alongside them if it comes to that — but do not groan inwardly.

— Enchiridion 16

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