Who said "Moral strength is a muscle that grows by use."?

This quote is attributed to Émile Zola.

Moral strength is a muscle that grows by use. - Émile Zola

"Moral strength is a muscle that grows by use."

Émile Zola Strength

Novelist French 1840–1902

"The only way to deal with an unfree world is to become so absolutely free that very existence is an act of rebellion."

"I am my brother's brother, and I feel that I cannot be happy while he is wretched."

"Life is a beautiful struggle."

"The artist's task is to save the soul of mankind."

"Strength comes from within."

"A woman should be proud of who she is."

"A strong woman can overcome anything."

"The only thing I'm good at is killing."