The character Jubal Harshaw says this line in Stranger. Interestingly, in the "Uncut" version of the book, the character does not speak this line. Many argue that Harshaw is the true main character of the book, as he consistently proffers the philosophies another character must deal with.
The author's Journal, excerpted from The Thoreau you don't Know... (2009) by Robert Sullivan, page 87
Deeply in love with a woman named Ellen, whom his own brother John also loved, Henry David Thoreau penned this line in his journal in 1839. He later proposed to her in 1841, as did his brother, and both were rejected.
The Little Book of Bathroom Philosophy : Daily Wisdom from the Greatest Thinkers (2004) by Gregory Bergman, p. 50
The original, full quote reads "Quantum in te crescit amor, tantum crescit pulchritudo; quia ipsa charitas est animae pulchritudo," and was fully translated as "Since love grows within you, so beauty grows. For love is the beauty of the soul." Here Augstine points out the inherent beauty that takes root within us when we love.