Footnote 7 from

A Short History of Blacks As The Special Favorites of the Law: (Revisiting the Failure of Reconstruction and America’s Persistent Race Problem)

7]   The legal foundations of the “Peculiar Institution” are buried in three discreet clauses in the Constitution. The words “slavery” and “African” are understood.

The first reference was in regard to the valuation of slave men as three-fifths of a man for purposes of apportioning representatives to Congress.    See U.S. Const. art. I, § 2, cl. 3

The second authorized the unimpeded continuation of the slave trade until 1808– “migration or importation of such persons as any of the States now existing shall think proper to admit”  

See  U.S. Const. art. I, § 9, cl. 1

The third provided the constitutional basis for the Fugitive Slave Acts,

“No Person held to Service or Labour in one State, under the Laws thereof, escaping into another, shall, in Consequence of any Law or Regulation therein, be discharged from such Service or Labour, but shall be delivered up on Claim of the Party to whom such Service or Labour may be due.”  

U.S. Const. art. IV, § 2, cl. 3.