Article Three of the United States Constitution
Section 3 also makes it clear that treason must be an "overt act."  This means that to commit treason, a person must actually do something, not just plan to do it.  For example, if a group of people got together and planned to attack part of the United States, that would not be treason because they had not actually done anything yet to hurt the country.  If they did attack the United States, that would be an overt act, and it would be treason.