“Fear is the mind-killer”

More specifically, it's from what is known as the "litany against fear," which goes:

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.

In the book, the litany against fear is repeated by an elite sisterhood with superhuman intellect and powers, called the Bene Gesserit, who use it to calm and focus.