Comes from his speech to the Young Men's Lyceum of Springfield, Illinois, January 27, 1838, which can be found at http://www.constitution.org/lincoln/lyceum.htm. Lincoln is warning of the dangers of the increasing arbitrariness of mob violence in retribution for alleged crimes.

In the Mississippi case they first commenced by hanging the regular gamblers — a set of men certainly not following for a livelihood a very useful or very honest occupation, but one which, so far from being forbidden by the laws, was actually licensed by an act of the Legislature passed but a single year before.... Abstractly considered, the hanging of the gamblers at Vicksburg was of but little consequence. They constitute a portion of population that is worse than useless in any community; and their death...is never matter of reasonable regret with any one. If they were annually swept from the stage of existence by the plague or smallpox, honest men would perhaps be much profited by the operation.

Sounds like not a lot has changed in 200 years.