Origen, on Num 2:33

“And the Levites were not numbered among the children of Israel: for so the Lord had commanded Moses.”

“We gather from the book of Numbers that there may be something greater than Israel too. For there the whole of Israel is numbered and reckoned in twelve tribes, as under a fixed number. But the tribe of Levi, being of greater eminence than the others, is accounted extra to this number and never thought of as being one of Israel's number [of twelve].”

Imported from an open dataset — not yet checked against the printed edition.