Out of life’s school of war: What does not destroy me, makes me stronger.

[Aus der Kriegsschule des Lebens. — Was mich nicht umbringt, macht mich stärker.]

Friedrich Nietzsche (1844-1900) German philosopher and poet

Twilight of the Idols [Die Götzen-Dämmerung], “Maxims and Arrows [Sprüche und Pfeile]” #8 (1889) [tr. Hollingdale (1968)]
    (Source)

Alt. trans.:

• "From the military school of life. -- What does not kill me, strengthens me." [tr. Common (1896)]
• "From the Military School of Life: Whatever does not kill me, makes me stronger. [tr. Large (1998), "Maxims and Barbs"]
• "From life's school of war. -- What doesn't kill me makes me stronger." [tr. Norman (2005), "Arrows and Epigrams"]
• "From the military school of life. -- That which does not kill me, makes me stronger." [tr. Ludovici (1911), "Maxims and Missiles"]