I'm pretty sure you're talking about the "marrow of life" quote.
It goes like this:
" I went into the woods because I wanted to live deliberately. I wanted to live deep and suck out all the marrow of life ... to put to rout all that was not life; and not, when I came to die, discover that I had not lived."
And it would be a quote by Thoreau from his novel, Walden.