Hebrews 7:5-6
“And verily they that are of the sons of Levi who receive the office of Priesthood, have a commandment to take tithes of the people according to the law, that is, of their brethren, though they come out of the loins of Abraham.”
So great (he would say) is the superiority of the priesthood, that they who from their ancestors are of the same dignity, and have the same forefather, are yet far better than the rest. At all events they “receive tithes” from them. When then one is found, who receives tithes from these very persons, are not they indeed in the rank of laymen, and he among the Priests?
And not only this; but neither was he of the same dignity with them, but of another race: so that he would not have given tithes to a stranger unless his dignity had been great. Astonishing! What has he accomplished? He has made quite clear a greater point than those relating to faith which he treated in the Epistle to the Romans. For there indeed he declares Abraham to be the forefather both of our polity and also of the Jewish. But here he is exceeding bold against him, and shows that the uncircumcised person is far superior. How then did he show that Levi paid tithes? Abraham (he says) paid them. 'And how does this concern us?' It especially concerns you: for you will not contend that the Levites are superior to Abraham. “But he whose descent is not counted from them, received tithes of Abraham.”
Source: Homilies on the Epistle to the Hebrews (New Advent)