History of the Peloponnesian War
Thucydides
Thucydides, Vol. 1-4. Smith, Charles Foster, translator. London and Cambridge, MA: Heinemann and Harvard University Press, 1919-1923.
So Nymphodorus came to Athens, brought about the alliance with Sitalces, and got Sadocus son of Sitalces made an Athenian citizen; and he promised also to bring the war in Thrace to an end, saying that he would persuade Sitalces to send the Athenians a Thracian force of cavalry and targeteers.
Moreover, he brought about a reconciliation between Perdiccas and the Athenians, whom he persuaded to restore Therme[*](Thuc. 1.61.2.) to him. Perdiccas immediately joined forces with the Athenians under Phormio[*](Thuc. 1.64.2; Thuc. 1.65.3.) and took the field against the Chalcidians.
It was in this way that Sitalces son of Teres, king of the Thracians, became an ally of the Athenians, and also Perdiccas son of Alexander, king of the Macedonians.