The History of the Grecian War (1.90.1-1.90.3)

The Lacedaemonians, hearing what they went about, sent thither their ambassadors—partly because they would themselves have been glad that neither the Athenians nor any other had had walls, but principally as incited thereto by their confederates who feared not only the greatness of their navy, which they had not before, but also their courage showed against the Persians—and entreated them not to build their walls but rather to join with them in pulling down the walls of what cities soever without Peloponnesus had them yet standing, not discovering their meaning and the jealousy they had of the Athenians but pretending this:

that if the barbarian returned, he might find no fortified city to make the seat of his war, as he did of Thebes, and that Peloponnesus was sufficient for them all whereinto to retire and from whence to withstand the war.

But the Athenians, by the advice of Themistocles, when the Lacedaemonian ambassadors had so said, dismissed them presently with this answer, that they would presently send ambassadors about the business they spake of to Lacedaemon. Now Themistocles willed them to send himself to Lacedaemon for one, and that as speedily as they could; but such as were chosen ambassadors with him not to send away presently, but to stay them till the walls were so raised as to fight upon them from a sufficient height; and that all the men in the city, in the meantime, both they and their wives and children, sparing neither private nor public edifice that might advance the work but pulling all down whatsoever, should help to raise it.