Where can I find it in writing that a U.S. citizen cannot be refused re-entry into the U.S. after an extended period abroad?

There actually is no fundamental right of U.S. citizens to leave their country and return home again. Under current Supreme Court precedents, the right to travel abroad is merely an aspect of liberty that may be restricted within the bounds of due process.  Some have argued that the Citizenship Clause of the 14th Amendment grants this right, but no court has yet decided that issue.