United States Constitution

Article IV.
Section. 3. New States may be admitted by the Congress into this Union; but no new State shall be formed or erected within the Jurisdiction of any other State; nor any State be formed by the Junction of two or more States, or Parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress.

Declaration of Independence of 1776

Vermont Republic

Secession.net

Jefferson State on Wikipedia

A Successful One-Family Secession Movement