Galatians 5:9
5 For through the Spirit, by faith, we wait expectantly for the hope of righteousness. 6 For in Christ Jesus neither circumcision nor uncircumcision carries any weight—the only thing that matters is faith working through love. 7 You were running well; who prevented you from obeying the truth? 8 This persuasion does not come from the one who calls you! 9 A little yeast makes the whole batch of dough rise! 10 I am confident in the Lord that you will accept no other view. But the one who is confusing you will pay the penalty, whoever he may be. 11 Now, brothers and sisters, if I am still preaching circumcision, why am I still being persecuted? In that case the offense of the cross has been removed.
Berakhot 17a
Babylonian TalmudAfter his prayer, Rabbi Alexandri said the following: May it be Your will, Lord our God, that You station us in a lighted corner and not in a darkened corner, and do not let our hearts become faint nor our eyes dim. Some say that this was the prayer that Rav Hamnuna would recite, and that after Rabbi Alexandri prayed, he would say the following: Master of the Universe, it is revealed and known before You that our will is to perform Your will, and what prevents us? On the one hand, the yeast in the dough, the evil inclination that is within every person; and the subjugation to the kingdoms on the other. May it be Your will that You will deliver us from their hands, of both the evil inclination and the foreign kingdoms, so that we may return to perform the edicts of Your will with a perfect heart.
Notes and References
"... The parable of the leaven (13:33) confirms the view that Jesus chose startling images to convey the humble and mysterious nature of the kingdom. Leaven was a symbol of corruption in the Hebrew Scriptures (Exodus 12:17-20; 23:18; 34:18; Leviticus 2:11, 6:17; Hos 7:4), in Matthew (16:6, 11), and elsewhere in the New Testament (1 Corinthians 5:6-13; Galatians 5:9; see also Mishna 'Orla 2:8-12; Menachot 5:1-2; Genesis Rabbah 34:10; Babylonian Talmud Berakot 17a; Plutarch, Roman Questions 289F: “Yeast [leaven] is itself also the product of corruption, and produces corruption in the dough with which it is mixed; ... and altogether the process of leavening seems to be one of putrefaction”). What leaven meant to the first-century Jew has been obscured by its translation as “yeast,” as if yeast and leaven were one and the same thing. Leaven was produced by keeping back a piece of the previous week’s dough, storing it in suitable conditions, and adding juices to promote the process of fermentation. After several days the old dough was sufficiently fermented to be used in a large mass of dough to give it lightness. This homemade rising agent, however, was fraught with health hazards. If it became tainted, it would spread poison to the rest of the dough in baking, and that batch of dough could infect the next batch and so on (see Pliny, Natural History 18:26). For this reason, leaven became a symbol for the infectious power of evil ..."
Garland, David E. Reading Matthew: A Literary and Theological Commentary (pp. 151-152) Smyth & Helwys, 2001