History of the Peloponnesian War

For there were both Argilians that dwelt in the same city (now Argilus is a colony of the Andrians), and others, that contrived this, induced thereunto some by Perdiccas and some by the Chalcideans.

But above all the Argilians, being of a city near unto it, and ever suspected by the Athenians, and secret enemies to the place, as soon as opportunity was offered and Brasidas arrived (who had also long before dealt underhand with as many of them as dwelt in Amphipolis to betray it), both received him into their own city, and revolting from the Athenians, brought the army forward the same night as far as to the bridge of the river.