One ought to fast, watch, and labor to the extent that such activities are needed to harness the body's desires and longings; however, those who presume that they are justified by works pay no attention to the need for self-discipline but see the works themselves as the way to righteousness. They believe that if they do a great number of impressive works all will be well and righteousness will be the result. Sometimes this is pursued with such zeal that they become mentally unstable and their bodies are sapped of all strength. Such disastrous consequences demonstrate that the belief that we are justified and saved by works without faith is extremely foolish.

p. 73 - The Freedom of a Christian (1520)