Matthew 16:1-12
Jesus' ministry must be making quite the impact if we see the Pharisees and Sadducees come together to request a sign from Jesus. Jesus always threatens man made religion because He offers direct access to God. Jesus' ministry was filled with proofs that He was indeed the Messiah that the Scriptures foretold, so He rebukes the religious leaders for their willful hypocritical blindness.
As they headed to the other side of the Sea of Galilee, the disciples forgot to bring bread. When Jesus then warns them of the leaven of the Pharisees and Sadducees, they think He's reproving them for their forgetfulness. Jesus uses their misunderstanding to make it very clear that He doesn't depend on their resources, but that they are completely dependent upon Him. Jesus reproves the smallness of their faith, not the smallness of their resources. We should always remember the Jesus is all sufficient for the work He calls us to.