Only actions give life strength; only moderation gives it a charm.

[Nur Taten geben dem Leben Stärke, nur Maß ihm Reiz.]

Jean Paul Richter (1763-1825) German writer, art historian, philosopher, littérateur [Johann Paul Friedrich Richter; pseud. Jean Paul]

Titan, Jubilee 35, cycle 145 (1803) [tr. Brooks (1863)]
    (Source)

Often only the first part is given as a quotation (or even just as a "German proverb").

(Source (German)). Alternate translations:

Only deeds give strength to life, and only measure gives it charm.
[Source (1858)]

Only deeds give strength to life, only moderation gives it charm.
[Source (1896)]