“What you HABITUALLY THINK largely determines what you will ultimately become.”
Bruce Lee (1940–1973) Hong Kong-American actor, martial artist, philosopher and filmmaker
Possibly, but more likely derived from the Buddha: "All that we are is the result of what we have thought. The mind is everything. What we think, we become."
Source: Striking Thoughts (2000), p. 120; This probably derives from a Rosicrucian proverb, "As you think, so shall you become", which is itself probably derived from Proverbs 23:7 "As he thinketh in his heart, so is he."