Passage 10.16.3
τὸν δὲ ὑπὸ Δελφῶν καλούμενον Ὀμφαλὸν λίθου πεποιημένον λευκοῦ, τοῦτο εἶναι τὸ ἐν μέσῳ γῆς πάσης αὐτοί τε λέγουσιν οἱ Δελφοὶ καὶ ἐν ᾠδῇ τινι Πίνδαρος ὁμολογοῦντά σφισιν ἐποίησεν.
The object called the Omphalos ("Navel") by the Delphians, made of white stone, the Delphians themselves claim to be the center of the entire earth; and Pindar in one of his songs has composed words in agreement with their claim.