Dhammapada verse 35 and 36

35. Wonderful, indeed, it is to subdue the mind, so difficult to subdue, ever swift, and seizing whatever it desires. A tamed mind brings happiness.

36. Let the discerning man guard the mind, so difficult to detect and extremely subtle, seizing whatever it desires. A guarded mind brings happiness.

– Dhammapada III – Cittavagga: The Mind 35 – 36