Pausanias Description of Greece 1.28.4

[4]
  On descending, not to the lower city, but to just beneath the Gateway, you see a fountain and near it a sanctuary of Apollo in a cave. It is here that Apollo is believed to have met Creusa, daughter of Erechtheus.... when the Persians had landed in Attica Philippides was sent to carry the tidings to Lacedaemon . On his return he said that the Lacedacmonians had postponed their departure, because it was their custom not to go out to fight before the moon was full. Philippides went on to say that near Mount Parthenius he had been met by Pan, who told him that he was friendly to the Athenians and would come to Marathon to fight for them. This deity, then, has been honored for this announcement.

[4] As one descends, not to the lower city, but to just beneath the Propylaia [‘Gateway’], there is a fountain [ pēgē] and near it a sacred space [ hieron] of Apollo in a cave [ spēlaion]. It was here, according to customary-thinking [ nomizein], that Apollo had sex [ sun-genesthai] with Kreousa, daughter of Erekhtheus. [There seems to be a gap here in the text.] When the Persians [Medes] had landed in Attica, Philippides was sent as a messenger [from Marathon] to Sparta [Lacedaemon]. When he returned [to Marathon], he said that the Spartans [Lacedaemonians] had postponed sending out [any reinforcements], on the grounds that it was their custom not to go out to fight before the circle of the moon was full. Philippides went on to say that near Mount Parthenios he had been met by Pan, who told him that he is kindly-disposed [ eu-nous] to the Athenians and would come as their ally [ summakhos] to Marathon. This deity, then, gets honored [ tīmâsthai] in return for this message [ angeliā].