Passage 1.18.4
ἐντεῦθεν ἰοῦσιν ἐς τὰ κάτω τῆς πόλεως Σαράπιδός ἐστιν ἱερόν, ὃν Ἀθηναῖοι παρὰ Πτολεμαίου θεὸν ἐσηγάγοντο. Αἰγυπτίοις δὲ ἱερὰ Σαράπιδος ἐπιφανέστατον μέν ἐστιν Ἀλεξανδρεῦσιν, ἀρχαιότατον δὲ ἐν Μέμφει· ἐς τοῦτο ἐσελθεῖν οὔτε ξένοις ἔστιν οὔτε τοῖς ἱερεῦσι, πρὶν ἂν τὸν Ἆπιν θάπτωσι. τοῦ δὲ ἱεροῦ τοῦ Σαράπιδος οὐ πόρρω χωρίον ἐστίν, ἔνθα Πειρίθουν καὶ Θησέα συνθεμένους ἐς Λακεδαίμονα καὶ ὕστερον ἐς Θεσπρωτοὺς σταλῆναι λέγουσι.
As you go from here down into the lower parts of the city, there is a sanctuary of Serapis, a god whom the Athenians introduced from Ptolemy. Among the Egyptians, the most famous sanctuary of Serapis is in Alexandria, but the oldest is in Memphis. Into this sanctuary at Memphis neither foreigners nor even priests may enter until the time they bury Apis. Near the sanctuary of Serapis in Athens, there is a place where they say that Peirithous and Theseus made an agreement to journey first to Lacedaemon, and afterwards to the Thesprotians.