I don't know the source, but this is likely the quote he is referring to:

"Heterogeneity of stocks may lead to faction – at any rate until they have had time to assimilate. A city cannot be constituted from any chance collection of people, or in any chance period of time. Most of the cities which have admitted settlers, either at the time of their foundation or later, have been troubled by faction. For example, the Achaeans joined with settlers from Troezen in founding Sybaris, but expelled them when their own numbers increased; and this involved their city in a curse. At Thurii the Sybarites quarreled with the other settlers who had joined them in its colonization; they demanded special privileges, on the ground that they were the owners of the territory, and were driven out of the colony. At Byzantium the later settlers were detected in a conspiracy against the original colonists, and were expelled by force; and a similar expulsion befell the exiles from Chios who were admitted to Antissa by the original colonists. At Zancle, on the other hand, the original colonists were themselves expelled by the Samians whom they admitted. At Apollonia, on the Black Sea, factional conflict was caused by the introduction of new settlers; at Syracuse the conferring of civic rights on aliens and mercenaries, at the end of the period of the tyrants, led to sedition and civil war; and at Amphipolis the original citizens, after admitting Chalcidian colonists, were nearly all expelled by the colonists they had admitted."(1303A13)

Aristotle (trans. Ernest Barker and R. F. Stalley), Politics (Oxford: Oxford University Press, 1995)

Edit: Added citation