Article One of the United States Constitution
The first clause in this section prevents Congress from passing any law that would restrict the importation of slaves into the United States prior to 1808.  Congress could however, levy a Per capita duty of up to ten dollars for each slave imported into the country.  This clause was further entrenched into the Constitution by Article V, where it is explicitly shielded from constitutional amendment prior to 1808.  On January 1, 1808, the first day it was permitted to do so, Congress approved legislation prohibiting the importation of slaves into the United States.