Neither Herodotus and Thucydides (the
contemporary Greek chroniclers of the time) ever mention "molon labe"
The source for the quote is Plutarch, (writing in 1 AD --half a millennium after the event)
Пάλιν δὲ τοῦ Ξέρξου γράψαντος 'πέμψον τὰ ὅπλα' ἀντέγραψε 'μολὼν λαβέ'
(
to Xerxes demand, "hand over your arms," [Leonidas] retorted, "come and get them".)
Moralia, III, Apophthegmata Laconica
The lack of contemporary record opens the quote to suspicion.
While Plutarch (a Greek living in Roman times) may have been several centuries removed from the events at Thermopylae, we can assume that a heck of a lot more that neither Herodotus nor Thucydides set into writing was preserved as
oral tradition, and perhaps Plutarch stumbled upon an accurate bit of such.
Plutarch, is really hamstrung not only by the remove of several hundred years from the events he attempts to describe, but by his repeated, provable errors and outright fabrications in the course of his fawning 'comparisons' of the Romans to the mighty Greeks of old.
On the other hand, neither Herodotus nor Thucydides were actual eyewitnesses to the battle (obviously) and thus relied on local rumor and probably a few partial witnesses. Those two 'contemporary' chroniclers were furthermore subject to making severe errors,
and could even resort to outright invention when providing 'missing' detail:
Herodotus offers a bogus explanation for Xerxes' elite troops being called "Immortals" ---that there was always an instant replacement for any who fell in battle-- when Herodotus merely confused the similar-sounding Persian words for 'Companion' and 'Immortal'. Xerxes' elite troops were literally his companions everywhere.
So.. anyway, did Leonidas really utter those words 'molon labe'?.
Herodotus doesn't really mention it, but he still provides some great supporting evidence.
He relates the Spartan Dienekes waxing with such shade-loving wit.
Did Herodotus make that up?
Not likely. Herodotus seems to be the sort, all too inclined to inflate the bravery and brilliance of Athens at the expense of Sparta. (Thucydides was yet another 'proud' Athenian)
Now, if Dienekes could realy be so (there's no other way to put it)
laconic...
and if laconism is to be taken as a 'national characteristic' of the Lakonians
(or Lakedaimonians; or Spartans if you like)
...then why wouldn't a King among them possess a laconic wit of kingly proportions?