Gregory the Great — as excerpted in the Catena Aurea on Matthew 26:26

“(non occ.) It has given trouble to divers persons, that in the Church some offer unleavened and others leavened bread. The Roman Church offers unleavened, because the Lord took flesh without any pollution1; other2 Churches offer leavened bread, because the Word of the Father took flesh upon Him, and is Very God, and Very Man; and so the leaven is mingled with the flour. But whether we receive leavened or unleavened, we are made one body of the Lord our Saviour.”