Mishnayos Menachos Perek 5 Mishnah 1

All the meal offerings come to be offered as matza, with care taken to prevent leavening, except for ten loaves of leavened bread among the forty loaves that accompany the thanks offering, and the meal offering of the two loaves that are brought on the festival of Shavuot, as they come to be offered as leavened bread. The Sages disagree as to the manner in which those meal offerings are leavened. Rabbi Meir says: With regard to the leaven added to the dough to facilitate leavening, one separates [bodeh] part of the flour for the meal offerings from within the flour of the meal offerings themselves, causes it to become leaven, and leavens the meal offerings with it. Rabbi Yehuda says: That is also not the optimal manner in which to fulfill the mitzva, as aged leaven is a more effective leavening agent. Rather, one brings the leaven from another, aged, dough and places it into the measuring vessel, and then he adds flour until he fills the measuring vessel, to ensure the appropriate measure of a tenth of an ephah of flour. The Rabbis said to Rabbi Yehuda: That too is inappropriate, as in that manner the meal offering will either be lacking the requisite measure or be greater than the required measure, as the Gemara will explain.

כָּל הַמְּנָחוֹת בָּאוֹת מַצָּה, חוּץ מֵחָמֵץ שֶׁבַּתּוֹדָה וּשְׁתֵּי הַלֶּחֶם, שֶׁהֵן בָּאוֹת חָמֵץ. רַבִּי מֵאִיר אוֹמֵר, שְׂאֹר בּוֹדֶה לָהֶן מִתּוֹכָן וּמְחַמְּצָן. רַבִּי יְהוּדָה אוֹמֵר, אַף הִיא אֵינָהּ מִן הַמֻּבְחָר, אֶלָּא מֵבִיא אֶת הַשְּׂאֹר, וְנוֹתֵן לְתוֹךְ הַמִּדָּה, וּמְמַלֵּא אֶת הַמִּדָּה. אָמְרוּ לוֹ, אַף הִיא הָיְתָה חֲסֵרָה אוֹ יְתֵרָה: