Hebrews 7:4

Now consider how great this man was, unto whom even the patriarch Abraham gave the tenth of the spoils.

Cross-reference

Hebrews 7:6 adds that Melchizedek blessed Abraham, showing his superiority as a priest-king, expanding the argument.

Hebrews 7:9 explains that even Levi tithed through Abraham, deepening the contrast between the Levitical and Melchizedekian orders.

Genesis 14:20 is the direct source: Abraham gave a tenth to Melchizedek, the event that Hebrews 7:4 highlights.