It's actually explicitly in the Constitution:
"The President...shall have Power to grant Reprieves and Pardons for Offences against the United States, except in Cases of Impeachment."
Article II, Section 2
"The President...shall have Power to grant Reprieves and Pardons for Offences against the United States, except in Cases of Impeachment."
Article II, Section 2
Actually, murder is a federal crime, see 18 USC Secs. 1111, 1114, and 1118-1121.
https://www.law.cornell.edu/uscode/text/18/part-I/chapter-51
> “The President . . . shall have Power to grant Reprieves and Pardons for Offenses against the United States, except in Cases of Impeachment.” Thus, the President’s authority to grant clemency is limited to federal offenses and offenses prosecuted by the United States Attorney for the District of Columbia in the name of the United States in the D.C. Superior Court. An offense that violates a state law is not an offense against the United States.
https://www.justice.gov/pardon/frequently-asked-questions-co...