Where is it stated that the President has the power of pardon and reprieve?

This power is granted in the US Constitution, article II, article 3, paragraph 1. , to wit :

"The President shall be Commander in Chief of the Army and Navy of the United States, and of the Militia of the several States, when called into the actual Service of the United States; he may require the Opinion, in writing, of the principal Officer in each of the executive Departments, upon any subject relating to the Duties of their respective Offices, and he shall have Power to Grant Reprieves and Pardons for Offenses against the United States, except in Cases of impeachment."