Doctrine and Covenants 59:23
I really like the idea of "peace in this life" as a reward. We aren't often promised specific blessings that we know will apply to our earthly lives, and often we see earthly prosperity that doesn't seem to be tied to righteousness at all. And that's okay. We're focusing on the long term, right? :) Amazingly cool, then, that we are promised peace now.
Wait, though... when do we get it? I want it today! :) ... I think the blessing is a byproduct of righteousness and faith. We feel peace when we trust God that things will work out, even when they seem bad. In Mormon 9:14 it tells us that "he that is happy shall be happy still; and he that is unhappy shall be unhappy still." ... I think that is sort of like this peace that we are promised. It comes from the way we live and learn to look at things. It isn't something that God lays over us like a blanket, but something he plants within us where we can always draw on it.
Today, let's do the works of righteousness, and learn how to be at peace.