Num 18:20

“Indeed, do you wish to know what the difference is between the priests of God and the priests of Pharaoh? Pharaoh grants lands to his priests. The Lord, on the other hand, does not grant his priests a portion in the land but says to them, "I am your portion." You therefore who read these words, observe all the priests of the Lord and notice what difference there is between the priests, lest perhaps they who have a portion in the land and have time for earthly cares and pursuits may appear not so much to be priests of the Lord as priests of Pharaoh. For it is Pharaoh who wishes his priests to have possessions of lands and to work at the cultivation of the soil not of the soul, to give attention to the fields and not to the law. But let us hear what Christ our Lord admonishes his priests: "He who has not renounced all he possesses," he says, "cannot be my disciple."”

“For all this people had twelve tribes after the number of the twelve sons of Jacob. What we call tribes are as it were distinct houses and congregations of people. This people, I say, had twelve tribes, out of which twelve tribes one tribe was Judah, out of which came the kings. There was another tribe, Levi, out of which came the priests. To the priests serving the temple no land was allotted. So it was necessary that among twelve tribes all the land of promise should be shared. Having separated one tribe of higher dignity, the tribe of Levi, which was of the priests, there would have remained eleven, unless by the adoption of the two sons of Joseph the number twelve were completed.”