I found what I was looking for.

This is Herodotus relating a story told to him about people crossing the Sahara.

[The Nasamonians] chose by lot five of their number to go to the uninhabited desert regions of Libya in order to find out whatever they could about them ... these youths were sent off by their peers well equipped with water and food, and they came first to the inhabited regions, and, passing beyond these, arrived at a land full of wild animals; then, making their way through the desert, they proceeded toward the west and, after crossing an extensive tract of sandy desert for many days, they at last saw trees growing on a plain. As they approached to pick the fruit hanging from the trees, they were attacked and taken captive by little men, that is, people shorter in stature than men. Neither the Nasamonians nor their captors could understand each other's language. They were led through vast swamps, an once they had passed through these, they arrived at a city in which all the people were the same height as their captors and had black skin. Alongside the city was a large river, flowing from the west toward the sunrise, in which crocodiles were visible.

... the Nasamonians returned homed and [claimed] that the people they had reached were all sorcerers.

The Histories by Herodotus, 2.32 [1]

I could have sworn there was at least one other mention of Sub-Saharan peoples but I guess I'll have to keep looking. Either way I find it fascinating to think that people were doing things like circumnavigating Africa in the 600s BCE.

[1] Herodotus, Robert B. Strassler, and Andrea L. Purvis. The Landmark Herodotus: The Histories. New York: Pantheon, 2007. Print. (link)