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.
The earliest traces of slavery actually surprisingly only dates back to the Moors, as the word "slave" derives from the word Slav.
The word slave is derived from the ethnonym (ethnic name) Slav. It arrived in English via the Old French sclave. In Medieval Latin the word was sclavus and in Byzantine Greek σκλάβος. Use of the word arose during the Early Medieval Period, when Slavs from Central and Eastern Europe (Saqaliba) were frequently enslaved by Moors from the Iberian Peninsula and North Africa.
However forced labour is thought to date back 11,000 years, possibly beginning with American indigenous peoples.
Slavery is rare among hunter-gatherer populations because it requires economic surpluses and a high population density to be viable. Thus, although it has existed among unusually resource-rich hunter gatherers, such as the American Indian peoples of the salmon-rich rivers of the Pacific Northwest Coast, slavery became widespread only with the invention of agriculture during the Neolithic Revolution about 11,000 years ago.
The earliest known example which i personally can find, comes from around 2600bc, when a Pharoah named Sneferu invaded Nubia and Libya in order to obtain slaves to build three pyramids, one of which is now one of the seven wonders of the ancient world.
To enable Sneferu to undertake such massive building projects, he would have had to secure an extensive store of labour and materials. According to Guillemette Andreu, this is where the king's foreign policy played a large part. Sneferu's conquests into Libya and Nubia served two purposes: the first goal was to establish an extensive labour force, and the second goal was to gain access to the raw materials and special products that were available in these countries. This is alluded to in the Palermo Stone:
One of the seven wonders
which would lead to Khufu's Great Pyramid, which would be seen as the pinnacle of the Egyptian Old Kingdom's majesty and splendour, and as one of the Seven Wonders of the Ancient World.
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....