Article Two of the United States Constitution
This clause says the President is the head of the Executive Branch.  It also mentions the Vice President, though the Constitution does not give him any executive powers.  However, the Constitution does say that the President and Vice President must be elected at the same time, for the same term (amount of time), and by the same constituency.  The Founding Fathers wanted to make sure the Executive Branch would survive, and stay independent, if the Vice President had to become the President.