Article Two of the United States Constitution
The Constitution says clearly that officials can be impeached for treason or bribery.  But it does not say exactly what "High crimes and Misdemeanors" includes or does not include.  "High crimes" is a phrase from old British law that means crimes committed by people with political power.  However, in British history, "high crimes and misdemeanors" could include many different things (even losing a ship by not tying it up).  After the Constitution was written, Alexander Hamilton explained in The Federalist Papers what kind of causes for impeachment the Founders had in mind: "offences which [come] from the abuse or violation of some public trust.  [They cause] injuries done immediately to the society itself."