Sure, no problem. It's right there in the Tenth Amendment:
Amendment 10Unless you can point out where the power of approving secession was delegated to the United States or prohibited to the states, then it is a power they retain individually.The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.
Congressional approval is required for a state to be admitted in the first place. Once admitted, congressional approval is required to combine two states or split a state into parts. Congressional approval is required to change the border of a state by a fraction of an inch. Implied in this is the need for congressional approval to leave since approval is needed for every other change in status.
States cannot, without consent of Congress, lay duties on imports from states. They cannot cut off water from states, form agreements or compacts against states, refuse to recognize a state's acts or proceedings, refuse to turn over criminals wanted in other states, form alliances, raise armies, all actions which, if done unilaterally, can have a negative impact on the interests of other states. If they cannot do them unilaterally while a part of the U.S. then implied in that is that they lack the power to do so merely by leaving as well.
Let us assume for a moment that a civil secession would not automatically be an act of treason or insurrection. This is not what occured. Rather, immediately uppon secedeing, Federal weaspons stores were sacked, an act of insurrection. virtually all reb3ell governments waged war upon citizens of those counties unwilling to secede, setting the precedent for a Federal Invasion to restore order.