It is by going down into the abyss
that we recover the treasures of life.

Where you stumble,
there lies your treasure.

The very cave you are afraid to enter
turns out to be the source of
what you are looking for.
The damned thing in the cave
that was so dreaded
has become the center.

Joseph Campbell (1904-1987) American writer, professor of literature

In Diane K. Osbon, ed., Reflections on the Art of Living: A Joseph Campbell Companion, “In the Field” (1991)
    (Source)

Quoted extensively, and mis-cited to a variety of Campbell's published works. I have not been able to confirm a primary source for it.