Matthew 15:1-20
In this passage Jesus is confronted by the Pharisees from Jerusalem because His disciples ate with unwashed hands. The Pharisees were professional fault-finders, and seemed to be more than willing to over look the mighty works that were being accomplished in Jesus ministry simple because their traditions were being ignored.
Jesus rebukes the Pharisees for elevating man's tradition above the Word of God, and goes on to teach the multitude about the true source of defilement. The Pharisees were extremely concerned with external defilement, but were completely neglectful regarding inward defilement.
Jesus teaches us very plainly that food enters our stomach, and not our heart, so food does not defile us. The source of defilement is our sinful hearts, and since defilement is an inward problem, it requires an inward solution. Human hands can't cleanse the human heart, but the blood of Jesus can!