疑心暗鬼
(Japanese: "Suspicion breeds suspicion" - literally, "Suspicious mind, dark devil")
Traditional Japanese proverb/saying (kotowaza), meaning that once you begin to harbour suspicions, they feed upon themselves, and you begin to see shadows and enemies everywhere, whether they are real or not.
A worthwhile concept to keep in mind, in these times.
This is a characteristically abbreviated form of the full idiom, by the way - the Japanese have a tendency to shorten any stock phrases. The full version is: 疑心暗鬼を生ずる (gishin'anki wo shouzuru), "A suspicious mind breeds dark devils".