First of all in most Empires, and even non-Empires like the ancient Greek city-states slavery was a direct child of war and the slave trade was heavily dependent on the getting prisoners of war. read more
As they didn't have written records but instead rely on oral history we can't be sure but it's likely that references to slavery would appear in some of their myths and legends as the freed slave made good is a very popular mythic archetype. read more