You're probably thinking of Jesus' words in Luke 17:21, where He said that "the kingdom of God is within you." These words are often misunderstood, however; Jesus was speaking only to His disciples, reminding them that God Himself now had come to live within them by His Holy Spirit.
You are right, up to a point: We will only know true and lasting peace in our hearts when we come to know God. But have you ever asked yourself why we don't already have that peace? Why do we seem to be at war within ourselves, and why are we so unsettled and restless? Why can't we get along with other people, and why can't we even get along with ourselves? In other words, what is our real problem? What cuts us off from God?
The Bible puts our problem in one word: sin. Sin has cut us off from God—and until our sin problem is solved, we will never know lasting peace. This is why looking within ourselves isn't enough, for we can never overcome our sin problem in this way. Much as we hate to admit it, the Bible's diagnosis is correct: "The heart is deceitful above all things and beyond cure. Who can understand it?" (Jeremiah 17:9).
This is why we need Christ—and this is why Christ came into the world. Don't be satisfied with anything less than Him, for only He can cleanse us from our sins, and only He can restore us to God. Open your heart and life to Him today.