Hebrews 7:5

James Moffatt - New Testament

Those sons of Levi who receive the priestly office are indeed ordered by law to tithe the people (that is, their brothers), although the latter are descended from Abraham;

Cross References

Hebrews 7:10

for he was still in the loins of his father when Melchizedek met him.

Hebrews 5:4

Also, it is an office which no one elects to take for himself; he is called to it by God, just as Aaron was.