Passage 2.37.4
ταῦτα μὲν δὴ ἀπέφαινεν οὕτως ἔχοντα, τῆς δὲ Ἀμυμώνης πέφυκεν ἐπὶ τῇ πηγῇ πλάτανος· ὑπὸ ταύτῃ τὴν ὕδραν τραφῆναι τῇ πλατάνῳ φασίν. ἐγὼ δὲ τὸ θηρίον πείθομαι τοῦτο καὶ μεγέθει διενεγκεῖν ὑδρῶν ἄλλων καὶ τὸν ἰὸν οὕτω δή τι ἔχειν ἀνίατον ὡς τὸν Ἡρακλέα ἀπὸ τῆς χολῆς αὐτοῦ τὰς ἀκίδας φαρμακεῦσαι τῶν ὀιστῶν· κεφαλὴν δὲ εἶχεν ἐμοὶ δοκεῖν μίαν καὶ οὐ πλείονας, Πείσανδρος δὲ ὁ Καμιρεύς, ἵνα τὸ θηρίον τε δοκοίη φοβερώτερον καὶ αὐτῷ γίνηται ἡ ποίησις ἀξιόχρεως μᾶλλον, ἀντὶ τούτων τὰς κεφαλὰς ἐποίησε τῇ ὕδρᾳ τὰς πολλάς.
These then he described as being so. Beside the spring of Amymone there grows a plane-tree; beneath this plane-tree, they say, the Hydra was nurtured. For my part, I am convinced that this beast surpassed other water-serpents in size and had venom so deadly that Heracles used its gall to poison the tips of his arrows. It seems to me that it had only one head, and not several; but Peisander of Camirus, wishing the creature to appear more terrifying and intending thereby to make his poetry more impressive, has given the Hydra many heads instead of one.