The House of Shammai say: leaven in the volume of an olive, and chametz in the volume of a date. And the House of Hillel say: both this and that in the volume of an olive. What is the reasoning of the House of Shammai? If so, let the Merciful One write "chametz" and not require "leaven," and I would have said: if chametz, whose leavening is not severe, is at an olive's volume, then leaven, whose leavening is severe, all the more so. Why then did the Merciful One write "leaven"? To tell you that the measure of this is not like the measure of that. And the House of Hillel: both are necessary. For if the Merciful One had written only "leaven," I would say it is because its leavening is severe, but chametz, whose leavening is not severe, say no. And if He had written only "chametz," I would say it is because it is fit to eat, but leaven, which is not fit to eat, say no. Therefore He teaches us both. And the House of Shammai do not hold that which Rabbi Zeira said: the verse opened with leaven and concluded with chametz, to tell you this is leaven and this is chametz. As to eating, all agree they do not differ; where they differ is as to removal. The House of Shammai hold we do not derive removal from eating, and the House of Hillel hold we do. "You shall eat nothing leavened" and so on. Why is this said? Because it says, "for whoever eats anything leavened shall be cut off," from this I know only chametz; its mixture, from where? Scripture says, "You shall eat nothing leavened."
The Measure of Leaven and Chametz in the Houses of Shammai and Hillel
Yalkut Shimoni on Torah 204:1
בֵּית שַׁמַּאי אוֹמְרִים, שְׂאוֹר בִּכְזַיִת וְחָמֵץ בִּכְכוּתֶבֶת. וּבֵית הִלֵּל אוֹמְרִים, זֶה וָזֶה בִּכְזַיִת. מַאי טַעְמָא דְּבֵית שַׁמַּאי, אִם כֵּן לִכְתּוֹב רַחֲמָנָא חָמֵץ וְלָא בָּעֵי שְׂאוֹר, וַאֲנָא אֲמִינָא, וּמַה חָמֵץ שֶׁאֵין חִמּוּצוֹ קָשֶׁה בִּכְזַיִת, שְׂאוֹר שֶׁחִמּוּצוֹ קָשֶׁה לֹא כָל שֶׁכֵּן, שְׂאוֹר דְּכָתַב רַחֲמָנָא לָמָּה לִי, לוֹמַר לְךָ, שֶׁשִּׁעוּרוֹ שֶׁל זֶה לֹא כְשִׁעוּרוֹ שֶׁל זֶה. וּבֵית הִלֵּל, צְרִיכֵי, דְּאִי כָתַב רַחֲמָנָא שְׂאוֹר, מִשׁוּם דְחִמּוּצוֹ קָשֶׁה, אֲבָל חָמֵץ שֶׁאֵין חִמּוּצוֹ קָשֶׁה אֵימָא לֹא. וְאִי כָּתַב רַחֲמָנָא חָמֵץ, מִשּׁוּם דְּרָאוּי לַאֲכִילָה, אֲבָל שְׂאוֹר דְּאֵין רָאוּי לַאֲכִילָה אֵימָא לֹא, קָא מַשְׁמַע לָן. וּבֵית שַׁמַּאי, לֵית לְהוּ לְהָא דְאָמַר רַבִּי זֵירָא פָּתַח הַכָּתוּב בִּשְׂאוֹר וְסִיֵּם בְּחָמֵץ לוֹמַר לְךָ זֶהוּ שְׂאוֹר זֶהוּ חָמֵץ. לְעִנְיַן אֲכִילָה כּוּלֵי עַלְמָא לֹא פְּלִיגֵי, כִּי פְּלִיגֵי לְעִנְיַן בִּעוּר, בֵּית שַׁמַּאי סָבְרֵי לֹא יַלְפִינָן בִּעוּר מֵאֲכִילָה,. וּבֵית הִלֵּל. כָּל מַחְמֶצֶת לֹא תֹאכֵלוּ וְכוּ' לָמָּה נֶאֱמַר, לְפִי שֶׁנֶּאֱמַר כִּי כָּל אֹכֵל מַחֲמֶצֶת וְנִכְרְתָה, אֵין לִי אֶלָּא חָמֵץ, תַּעֲרָבְתּוֹ מִנַּיִן, תַּלְמוּד לוֹמַר כָּל מַחֲמֶצֶת לֹא תֹאכֵלוּ.