The earliest example I know is from Sumer. That would be the earliest example of institutionalized slavery, because that's (one of) the earliest forms of urbanized civilization. However, what's the story gleaned from pre history, can we give a date for when the targeted raid with the explicit goal to subjugate foreigners starts to appear?
Slavery became only possible when people got the means to keep slaves, which was after the neolithic revolution. Slaves need feeding, some care, they need to be properly locked up and guarded. That's a pretty big resource drain for hunter-gatherers.
Of course hunter-gatherers had plenty of nasty/dangerous jobs they'd love to give to slaves. But they lacked the means to do it. When people started to settle down and became agriculturalists they got the capacity to keep slaves.
This is in a nutshell what Guns, Germs and Steel goes into with much more detail.
@T.E.D.: herding societies are pastoralists. They have -usually- less resources, consequentially have less resources to keep slaves. Doesn't say that they didn't keep slaves. They kept less slaves because they lacked the resources to keep more.
It all depends on how you define slavery.
If by slavery you mean, prisoners with jobs. Then I am sure it predated history and global.
But if by slavery you mean, people are treated as livestock, bred like livestock, sold like livestock, and even their offsprings are slave.... then that is actually a tradition that is certainly not global. That kind of slavery seems to be a mostly Mediterranean and Middle Eastern tradition. And it seems the justification of such practices was generally based on religious principles....