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.
