Article Three of the United States Constitution
It also says that there are only two ways for a person to be convicted of treason.  Two different witnesses must say in court that they saw the person do that overt act.  If there are not two witnesses, then the person must admit in court that they committed treason.  If neither of these things happen, the person cannot be found guilty of treason.  Parts of this rule came from a British law called the Treason Act 1695.