Thought I'd give someone else time to think of an answer. (Actually, was a bit stumped, though I know I have heard this one several times before in the past).
Without checking any of my books, I'll just quess and say it might be from Go Rin no Sho (The book of five rings) by Miyamoto Musashi. That's probably wrong, though. If he had said it, he would have most likely gotten it from an older source. Any hints coming our way?
As far as the meaning behind the quote, I think that I'll use another quote from Takuan Soho to explain it:
It is the very mind itself that leads the mind astray; of the mind, do not be mindless...
All streams flow to the sea because it is lower than they are. Humility gives it its power.