Matthew 16:12
Then they understood that he did not tell them to beware of the leaven of bread, but of the teaching of the Pharisees and Sadducees. The disciples' eventual understanding: the leaven is the teaching. The teaching of the Pharisees (tradition over commandment, outward observance without heart-alignment) and the Sadducees (denial of resurrection, skepticism about spiritual reality) are the pervasive influences that the disciples must guard against.