1 Corinthians 5:6-8

Your boasting is not good. Do you not know that a little leaven leavens the whole lump of dough? Clean out the old leaven so that you may be a new lump, just as you are in fact unleavened. For Christ our Passover also has been sacrificed. Therefore let us celebrate the feast, not with old leaven, nor with the leaven of malice and wickedness, but with the unleavened bread of sincerity and truth.