Final answer:
The leaven of the Pharisees refers to their hypocrisy in the Bible.
Step-by-step explanation:
In the Bible, in Matthew 16:6, Jesus warns his disciples, saying 'Beware ye of the leaven of the Pharisees, which is hypocrisy.' The Pharisees were a powerful Jewish group in the first century CE, who emphasized strict adherence to the laws of Moses.