27 United States Code, Chapter 8, Subchapter I, Section 205
(a) Exclusive outlet
[It shall be unlawful ...] To require, by agreement or otherwise, that any retailer engaged in the sale of distilled spirits, wine, or malt beverages, purchase any such products from such person to the exclusion in whole or in part of distilled spirits, wine, or malt beverages sold or offered for sale by other persons in interstate or foreign commerce, if such requirement is made in the course of interstate or foreign commerce, or if such person engages in such practice to such an extent as substantially to restrain or prevent transactions in interstate or foreign commerce in any such products, or if the direct effect of such requirement is to prevent, deter, hinder, or restrict other persons from selling or offering for sale any such products to such retailer in interstate or foreign commerce.
Source: U.S. Code Online via GPO Access