Unleavened bread
Yeast, or leaven, is symbolic of sin.
That is why unleavened bread is consumed at Passover, and at Communion.
The leaven represents sin, and the Passover, who is Christ,
is without sin so unleavened bread represents Christ, Who was without sin.
The Communion bread also represents the body of Christ, which was without sin.
So it too must be unleavened bread.
The common bread of Old Testament times was made with leaven.
Such bread was acceptable as wave offerings for the priests
and as loaves to accompany the peace offerings
Leviticus 7.11-13 Leviticus 23.17
However, bread made with leaven or honey,
was never to be used as offerings to be burned on the alter.
In the New Testament, leaven is a symbol of any evil influence which,
if allowed to remain, can corrupt the church.
Jesus warned His disciples against the leaven of the Pharisees, their teaching and hypocrisy