The History of the Grecian War (1.59.1-1.60.1)

The Athenian galleys, when they arrived in Thrace, found Potidaea and the other cities already revolted.

And the commanders of the fleet, conceiving it to be impossible, with their present forces, to make war both against Perdiccas and the towns revolted, set sail again for Macedonia, against which they had been at first sent out, and there staying, joined with Philip and the brothers of Derdas that had invaded the country from above.

In the meantime after Potidaea was revolted, and whilst the Athenian fleet lay on the coast of Macedonia, the Corinthians, fearing what might become of the city and making the danger their own, sent unto it, both of their own city and of other Peloponnesians which they hired, to the number of sixteen hundred men of arms and four hundred light armed.