Uranium ore is the raw precursor needed for both the production of enriched uranium (uranium in which the rare U-235 isotope has been increased to artificially high levels) and plutonium (which is formed by putting uranium into a nuclear reactor and exposing it to neutrons).

The US acquired approximately 10,000 tons of raw uranium ore under the auspices of the Manhattan Project. 72% of that came from the Belgian Congo, 14% from the the Colorado Plateau region, 9% from sources in Canada, and 5% from other miscellaneous sources. The Congo ore was particular valued as the amount of uranium per mass of ore was unusually high: it could be has high as 75% uranium. By contrast, the Canadian ores were at most 30% uranium per mass of ore, and the American ores were 0.25% uranium (not a typo).

So it is fair to say that the majority of the uranium that made the fuel for both the Little Boy and Fat Man bombs came from the Congo.

As for the labor conditions: the Belgian Congo was not the same thing as the Belgian Free State, which involved explicit slavery/forced labor and is what one thinks of primarily when thinking of the crimes committed against the Congolese people. The labor conditions were certainly not great. But was it slavery? I have not seen anything to indicate that; I suspect the miners were compensated, however poorly (and without adequate health protections, etc.). Certainly likely to have been awful labor conditions, though — as was certainly the case for further uranium mining operations in Africa perpetuated by colonial powers. On the latter, see Gabrielle Hecht, Being Nuclear: Africans and the Global Uranium Trade.