Article 1, Section 9, Clause 5
James Madison to Thomas Jefferson24 Oct. 1787Papers 10:214
Begging pardon for this immoderate digression I return to the third object above mentioned, the adjustments of the different interests of different parts of the Continent. Some contended for an unlimited power over trade including exports as well as imports, and over slaves as well as other imports; some for such a power, provided the concurrence of two thirds of both Houses were required; Some for such a qualification of the power, with an exemption of exports and slaves, others for an exemption of exports only. The result is seen in the Constitution. S. Carolina & Georgia were inflexible on the point of the slaves.
The Founders' Constitution
Volume 3, Article 1, Section 9, Clause 5, Document 2
The University of Chicago Press
The Papers of James Madison. Edited by William T. Hutchinson et al. Chicago and London: University of Chicago Press, 1962--77 (vols. 1--10); Charlottesville: University Press of Virginia, 1977--(vols. 11--).