Inner strength as a great river

I was watching the old movie "Kung Fu" (1986) and there is a really interesting moment near the end where Caine is lectured by Master Po who speaks to the effect of the following: "Do not go in fear, fear is eternal darkness, but go instead with inner strength which is like a deep river into which all streams flow; it increases, always moving forwards and soon there is nothing that can stand in its way." It seems like an empty sentiment at first, but on reflection there is something in this statement that makes me think it must come from some long lived idea. Is this just a screen writer's fancy or some reference to an old Buddhist idea?