I think you're thinking of the Mentat chant.
It is by will alone I set my mind in motion. It is by the juice of Sapho that thoughts acquire speed. The lips acquire stain. The stains become a warning. It is by will alone I set my mind in motion.Another handy quote from Dune:
I must not fear. Fear is the mind-killer. Fear is the little-death that brings total obliteration. I will face my fear. I will permit it to pass over me and through me. And when it has gone past I will turn the inner eye to see its path. Where the fear has gone there will be nothing. Only I will remain.