If you ask the internet, it will tell you -- over and over -- that Augustine described a sinner as homo curvatus in se (or in se curvatus, or se incurvatus; same diff.). It will not, however, tell you where Augustine might have said that. A search of Augustine's online opera omnia reveals virtually nothing, except a remote possibility that the phrase has its inspiration in Ps. 38:6 (Vg. 37:7). (There, however, Augustine clearly relates miser sum et curvatus to the humility of Christ).
Matt Jenson has written a dissertation-cum-book on the subject of sin-as-curvature in Augustine, Luther and Barth, of which we can skim part at Amazon. (This is a great idea for a book, and we look forward to reading it.) He mentions that Anders Nygren located the image "in two of Augustine's expositions of the psalms and in one of his sermons," and offers a citation to Agape and Eros (Harper & Row: 1969), p. 485 n.6. Since it's under copyright, Google books will only provide us with a snippet of the Nygren book, and it doesn't include the reference.
So. Does a kindly reader have a moment to skim one or the other of these books, and give us an idea of where the late bishop of Hippo used the historically-important phrase in question? Just because questions like this keep us awake at night.