1 Corinthians 5:6-8

6Your boasting is not good. Do you not know that a little leaven works through the whole batch of dough? 7Get rid of the old leaven, that you may be a new unleavened batch, as you really are. For Christ, our Passover lamb, has been sacrificed. 8Therefore let us keep the feast, not with the old bread, leavened with malice and wickedness, but with the unleavened bread of sincerity and of truth.