Where in the Constitution is that found?
The Constitution says nothing about secession, one way or another -- so every argument has to look for other Constitutional language and then claim: well, this means x and that means y and therefore my position for / against secession is true.
But the real truth is, our Founders considered their Constitution -- just like the previous Articles of Confederation -- to be "perpetual" and "forever."
Of course, they realized it might have to be dissolved, at some time in the future.
But that would have to be like a marriage divorce -- could not be for no-reason or "at pleasure", had to be by "mutual consent," or from "abuse" and "usurpations" having that same effect.
So the Founders provided no method for leaving the Union, though logically it would be identical to that for a state entering the Union -- namely with the approval of Congress.
What happened in historical fact was, there were no actual "usurpations" or "abuses" and no "mutual consent," so the Deep South first seceded "at pleasure" because of their fears of what an incoming anti-slavery Republican administration might do in the future.
The simple fact is that President Lincoln and Congress did not consider the Deep South's secession to be Constitutional, and when the South began shooting at Federal Forces, Lincoln declared an insurrection.
The Confederacy then declared war, and the rest, as they say, is history.