Art IV, Sec. 2, and 14th amendment.
Article IV, section 2 refers to a citizen, but does not really define one.
The citizens of each state shall be entitled to all privileges and immunities of citizens in the several states.
A person charged in any state with treason, felony, or other crime, who shall flee from justice, and be found in another state, shall on demand of the executive authority of the state from which he fled, be delivered up, to be removed to the state having jurisdiction of the crime.