The “proverb” is used in Jame Clavell’s 1975 novel Shōgun. In it on page 622 (of my pdf) Buntaro says, “It’s only a matter of waiting beside a river long enough for the bodies of your enemies to float by…”.
The entire paragraph is:
The vein on Buntaro’s forehead was throbbing and he was talking almost to himself. “They’ll pay. All of them. The traitors. It’s only a matter of waiting beside a river long enough for the bodies of your enemies to float by, neh? I’ll wait and I’ll spit on their heads soon, very soon. I’ve promised myself that.” He looked at her. “I hate traitors and adulterers. And all liars!”