AND THE PEOPLE TOOK THEIR DOUGH BEFORE IT WAS LEAVENED. On the strength of this is founded the precept that the leaven should be burned on the Passover Eve. "Leaven,' and "unleaven" symbolize the evil and the good inclinations in man.
For when Israel went out from Egypt they emerged from the dominion of the Egyptians, from that dominion which is called "leaven"; for the "evil inclination" operates in man and grows in him like leaven in the dough: it enters into him, and, little by little, extends its influence until his whole self is permeated by it
Soncino Zohar, Shemoth, Raya Mehemna, Page 40b
For when Israel went out from Egypt they emerged from the dominion of the Egyptians, from that dominion which is called "leaven"; for the "evil inclination" operates in man and grows in him like leaven in the dough: it enters into him, and, little by little, extends its influence until his whole self is permeated by it
Zohar 2:182a
According to some this was the prayer of R. Hamnuna, and R. Alexandri on concluding his prayer used to add the following: Sovereign of the Universe, it is known full well to Thee that our will is to perform Thy will, and what prevents us? The yeast in the dough...
Talmud, Berachot 17a