From The Federalist, Number 48: The Separation of Powers II written by James Madison aka "Publius" between 1787-1788.
"It is agreed on all sides, that the powers properly belonging to one of the departments ought not to be directly and completely administered by either of the other departments. It is equally evident, that none of them ought to possess, directly or indirectly, and overruling influence over the others, in the administration of their respective powers. It will not be denied, that power if of an encroaching nature, and that it ought to be effectually restrained from passing the limits assigned to it. After discriminating, therefore, in theory, the several classes of power, as they may in their nature be legislative, executive or judiciary, the next and most difficult task is to provide some practical security for each, against the invasion of the others. What this security ought to be, is the great problem to be solved."
For your self education enjoyment I present: Article I - The Legislative Branch