I am trying to locate a source in Augustine's Writings, quoted by John Donne in his sermon on Psalm 32:9 ("Be not as the Horse, or the Mule, who have no understanding").
Donne writes in his sermon: "And Saint Augustine carries these two qualities farre; He thinks that in this fierceness of the Horse, the Gentiles are represented, which ran farre from the knowledge of Christianity; And by the lazinesse of the Mule, the Jews, who came nothing so fast, s they were invited by their former helps, to the imbracing thereof."
I have checked, as best as I could, Augustine's _Confessions_ and _Expositions on Psalms_ but could not locate it.
See Confessions, Book Ten, chapter seven, and Augustine's Expositions on the Psalms at Psalm 32 and Psalm 34. I think that Exposition 34 is Donne's reference, but a closer reading than my cursory glance is well in order. I know that Augustine returns to this image throughout his literary corpus, but to the best of my knowledge these three spots are the most condensed. I hope this is of some help.
Good news! I believe that the expositions are in volume 37 of the PL. Best of luck with the Latin reading. Also, some standard translations of the fathers are on-line at http://www.ccel.org/fathers2/. It's helpful for its find function.