Genesis 14:20

“And blessed be the most high God, which hath delivered thine enemies into thy hand. And he gave him tithes of all.”

KJV

Meaning

Melchizedek praises God for the victory, and Abram gives him a tenth in response to the encounter.

Abram's tenth is a concrete act in this scene; this verse alone does not establish every later rule concerning tithing.

The movement from victory to praise and giving prevents military success from becoming self-glorification.