Aristotle's Metaphysics 981a19-20:
οὐ γὰρ ἄνθρωπον ὑγιάζει ὁ ἰατρεύων ἀλλ᾽ ἢ κατὰ συμβεβηκός, ἀλλὰ Καλλίαν ἢ Σωκράτην ... ᾧ συμβέβηκεν ἀνθρώπῳ εἶναι.
My translation:
For the doctor doesn’t cure a man, except accidentally, but [he cures] Callias or Socrates ... to whom it is happened to be a man.
Why ἀνθρώπῳ as a predicate is dative?
It is notable that in the Latin translation, the equivalent word is accusative:
cui esse hominem accidit
συμβαίνω in the sense "happen to" (section A.III.b of the LSJ entry) takes the dative for the person something happens to. In this sentence, the dative it takes is the relative pronoun ᾧ: "to whom it happens". The copula εἶναι requires that its predicate should appear in the same case as whatever word it's being equated with; most often this is the nominative, but when that word is in a different case for some reason, the predicate of εἶναι will still agree with it. So ᾧ is dative because of συμβέβηκεν, and ἀνθρώπῳ is dative because of ᾧ.
