In Matthew 16:6, 12 (RSV), Jesus warns His disciples:

"Take heed and beware of the leaven of the Pharisees and Sadducees."

Later, in Matthew 16:12, it is clarified:

"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."

Here, "leaven" is used metaphorically to represent the false doctrines and misleading teachings of the Pharisees and Sadducees, which could corrupt true faith.