I have a reference from George Gillespie, an opaque reference to Augustine. He says: "And that scandalous trespasses are understood, Matthew 18:19 (as Augustine, Tostatus, and many others have observed." No problem finding the Tostatus, but what work by Augustine addresses this? My searches so far on the Latin for that verse only turn up Speculum de evangelio Matthaei (or De Scriptura sacra speculum in Migne), which appears to simply be a compilation of Scripture texts; no commentary.