Aelian, Historical Miscellanies 2.21 = T 80 Kannicht
ἤρα δέ, φασι, τοῦ αὐτοῦ Ἀγάθωνος τούτου καὶ Εὐριπίδης ὁ ποιητής, καὶ τὸν Χρύσιππον τὸ δρᾶμα αὐτῶι χαριζόμενος λέγεται διαφροντίσαι. καὶ εἰ μὲν σαφὲς τοῦτο, ἀποφήνασθαι οὐκ οἶδα, λεγόμενον δ’ οὖν αὐτὸ οἶδα ἐν τοῖς μάλιστα.
They say that Euripides the poet loved this same Agathon and that he composed the play Chrysippus in his honour. Whether this is accurate, I am not able to demonstrate, but I know that it is said very frequently.