Matthew 16:12
Then they understood that He was not telling them to watch out for the yeast in [physical] bread, but for the teaching of the Pharisees and Sadducees.
Luke 12:1
About this time, while a crowd of many thousands had assembled, [so large that] people were trampling on one another, Jesus first began telling His disciples, "Watch out for the leavening [effect] of the Pharisees, which is hypocrisy.
Acts 12:3
When he realized that this act pleased the Jews, he proceeded to have Peter arrested also. [This happened] during the Festival of Unleavened Bread [See Exodus 12:15ff].
1 Corinthians 5:7-8
Get rid of the old yeast [i.e., of sin, and especially the unrepentant sinner], so that you can be a new batch of dough, without yeast in it [i.e., a godly church without unrepentant sinners in it] as [I know] you really are. For our Passover [Lamb], Christ, has already been sacrificed.
Galatians 5:12
I wish that those who are causing such a problem over wanting people to be circumcised would just mutilate themselves [while they are at it!].