When the disciples reached the other side, they had forgotten to bring any bread. Jesus said to them, “Watch and beware of the leaven of the Pharisees and Sadducees.” And they began discussing it among themselves, saying, “We brought no bread.” But Jesus, aware of this, said, “O you of little faith, why are you discussing among yourselves the fact that you have no bread? Do you not yet perceive? Do you not remember the five loaves for the five thousand, and how many baskets you gathered? Or the seven loaves for the four thousand, and how many baskets you gathered? How is it that you fail to understand that I did not speak about bread? Beware of the leaven of the Pharisees and Sadducees.” Then they understood that he did not tell them to beware of the leaven of bread, but of the teaching of the Pharisees and Sadducees.
Matthew 16:5-12 (ESV)