'Beware of the Leaven' in the Bible

Then Jesus said unto them, "Take heed, and beware of the leaven of the Pharisees, and of the Sadducees."

Why perceive ye not, then, that I spake not unto you of bread, when I said, 'Beware of the leaven of the Pharisees and of the Sadducees?'"

Then understood they, how that he bade not them beware of the leaven of bread: but of the doctrine of the Pharisees, and of the Sadducees.