'Beware of the Leaven' in the Bible

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

How is it that you do not understand that I did not speak to you about bread? But 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.