Here it is directly from the Gun Control Act of 1968, US Code Title 18, Part 1, Chapter 44, Section 922 (a)5:
(a) It shall be unlawful—
(5) for any person (other than a licensed importer, licensed manufacturer, licensed dealer, or licensed collector) to transfer, sell, trade, give, transport, or deliver any firearm to any person (other than a licensed importer, licensed manufacturer, licensed dealer, or licensed collector) who the transferor knows or has reasonable cause to believe does not reside in (or if the person is a corporation or other business entity, does not maintain a place of business in) the State in which the transferor resides; except that this paragraph shall not apply to
(A) the transfer, transportation, or delivery of a firearm made to carry out a bequest of a firearm to, or an acquisition by intestate succession of a firearm by, a person who is permitted to acquire or possess a firearm under the laws of the State of his residence, and
(B) the loan or rental of a firearm to any person for temporary use for lawful sporting purposes;
You can find it at: http://www.law.cornell.edu/uscode/html/uscode18/usc_sec_18_00000922----000-.html
There are big penalties and BATFE wouldn't have a sense of humor if they caught someone.