The Pharisees came and began to argue with him, seeking from him a sign from heaven, to test him. And he sighed deeply in his spirit, and said, “Why does this generation seek a sign? Truly, I say to you, no sign shall be given to this generation.” And he left them, and getting into the boat again he departed to the other side. Now they had forgotten to bring bread; and they had only one loaf with them in the boat. And he cautioned them, saying, “Take heed, beware of the leaven of the Pharisees and the leaven of Herod.”
–Mark 8:11-15