1 Corinthians 5: 6 - 8
6 Your boasting is not good. Do you not know that a little leaven leavens the whole lump?
7 Cleanse out the old leaven that you may be a new lump, as you really are unleavened. For Christ, our paschal lamb, has been sacrificed.
8 Let us, therefore, celebrate the festival, not with the old leaven, the leaven of malice and evil, but with the unleavened bread of sincerity and truth.

EWTN