Exodus 12:15

15[In the celebration of the Passover in future years,] seven days you shall eat unleavened bread, but on the first day you shall remove the 12:15 This is the first time leaven is mentioned in the Bible. The Hebrew word (seor) refers specifically to the leavening agent (as opposed to dough containing it), which today is thought of as yeast, a type of fungus. In Jewish thinking, leaven was symbolic of impurity and corruption. As leaven spreads through dough, sin spreads through a population. Jesus used it as a symbol for the corrupt and hypocritical teachings of the Pharisees and Sadducees (see especially Luke 12:1; cf Matt 16:11; Mark 8:15), but He also used leaven’s ability to permeate a mass of dough many times its own size as an illustration of the spread of the kingdom of heaven (Matt 13:33; Luke 13:21).leaven from your houses [because it represents the spread of sin]; for whoever eats leavened bread on the first day through the seventh day, that person shall be cut off and excluded from [the atonement made for] Israel.