Matthew 16:11
How is it that ye do not understand that I spake it not to you concerning bread, that ye should beware of the leaven of the Pharisees and of the Sadducees? How is it that ye do not understand that I spake it not to you concerning bread, that ye should beware of the leaven of the Pharisees and of the Sadducees?
Jesus was not speaking about literal or ordinary bread, nor about the disciples' lack of physical bread, but was speaking figuratively. The leaven to beware of is the teaching and doctrines of the Pharisees and of the Sadducees, a meaning the disciples came to understand.
That teaching about Jesus and religion could mislead through slanders and false ideas, superstition, religious show, self-righteous pride, worldliness, and unbelief. His reminders of the miraculous feedings show that the disciples need not fear going without bread.
That teaching is hypocrisy that presents human commandments and traditions as the word and commandments of God. It includes the Sadducees' rejection of the resurrection and the idea that souls pass into other bodies, which makes resurrection impossible, contrary to the teaching that life and death are set before people and that resurrection is essential to life. The warning applies to false teaching about the state of the dead, including ideas adopted from Greek and Roman beliefs.