According to google-sensei, your quote comes from Sun Tzu's art of war. If this is the case, the original quote would be in Chinese, not Japanese. Another source claims it's from a Chinese proverb, but starts off differently:
Tending the garden is a relaxing pastime, ...
I took the Chinese words for gardener and warrior and added "gardener in war" in quotes to a search, and nearly hit the google jackpot:
2 results that have the exact quote you're looking for:
花园里的战士好过战场上的园丁。
Huāyuán (garden) lǐ de (in the) zhànshì (warrior) hǎoguò (better than) zhànchǎng (battlefield) shàng de (on the) yuándīng (gardener).
(the grammar structure in Chinese is different from english, the translations are just there to show which words mean what)