I've seen the quote, "The New is in the Old Concealed, the Old is in the New Revealed" (such as a question here or the second answer here) or a variation, "The Old Testament is the New Testament concealed, and the New Testament is the Old Testament revealed" (such as in this answer) and I have typically seen this attributed to Augustine, though I haven't found its source. I have only found these two related but non-identical statements of St. Augustine from "On the Spirit and the Letter."
"This grace hid itself under a veil in the Old Testament, but it has been revealed in the New Testament" (from chapter 27)
This is the righteousness of God, which was veiled in the Old Testament, and is revealed in the New (from chapter 18)
Where does this quote come from?