"And the remainder of it" (Leviticus 6:9) - that which is valid and not that which is invalid. "Aaron and his sons shall eat" - this expanded its eating, [permitting it to be eaten] with any food they wish. "They shall eat it" - that they may eat ordinary food and priestly portions along with it when it is small in amount. "To Aaron and his sons" - to Aaron first, and afterward to his sons. To Aaron not in division, and to his sons in division: just as Aaron the High Priest eats without dividing, so too his sons who are High Priests eat without dividing. "It shall be eaten unleavened" - this is a positive commandment. Because it had been within the category of the permitted, and then became forbidden, and then returned and was permitted again, one might think it returns to its original permitted state [namely, optional]; therefore Scripture teaches "it shall be eaten unleavened" - it is a commandment. Similar to it: "her brother-in-law shall come in to her" (Deuteronomy 25:5) - it is a commandment. Because she had been within the category of the permitted, and then became forbidden, and then returned and was permitted again, one might think she returns to her original permitted state; therefore Scripture teaches "her brother-in-law shall come in to her" - it is a commandment. Who is the Tanna of that which our Rabbis taught, "her brother-in-law shall come in to her" is a commandment, and so forth? Rabbi Yitzchak bar Avdimi said: It is Abba Shaul, and this is what he means: "her brother-in-law shall come in to her" as a commandment. For at first she was to him within the category of the permitted - if he wished, he married her for the sake of her beauty; if he wished, he married her for the sake of marriage. Then she became forbidden, and returned and was permitted; one might think she returns to her original permitted state; therefore Scripture teaches "her brother-in-law shall come in to her" as a commandment. [Rava said: You may even say it accords with the Rabbis, and this is what he means: "her brother-in-law shall come in to her" as a commandment,] for at first she was to him within the category of the permitted - if he wished he married her, if he wished he did not marry her; then she became forbidden and returned and was permitted; one might think she returns to her original permitted state, that if he wished he married her and if he wished he did not. But she is already bound to him, so what would release her? Rather say: if he wished he married her, if he wished he performed chalitzah [the release ceremony]; therefore Scripture teaches "her brother-in-law shall come in to her" as a commandment. Now apply this to the opening clause: "it shall be eaten unleavened" as a commandment, for at first it was permitted to him, and so forth. According to Rava, who said it accords with the Rabbis, this is what it means: "it shall be eaten unleavened" is a commandment, for at first it was within the category of the permitted - if he wished he ate it, if he wished he did not eat it; then it became forbidden and returned and was permitted; one might think it returns to its original permitted state, that if he wished he ate it and if he wished he did not. But is it not written, "and they shall eat those things wherewith atonement was made" (Exodus 29:33), teaching that the priests eat and the owners are atoned for? Rather: if he wished he himself ate it, if he wished another priest ate it; therefore Scripture teaches "it shall be eaten unleavened" - it is a commandment. But according to Rav Yitzchak bar Avdimi, who said the whole matter accords with Abba Shaul, what two options are there here? If he wished he ate it as unleavened bread, if he wished he ate it boiled. But this boiled form - what is it like? If it is unleavened, it is already unleavened; and if it is not unleavened, surely the Merciful One said "unleavened"! In truth it is unleavened, and the reason the Merciful One repeated it for us is to make the matter indispensable.
The Unleavened Remnant Eaten by Aaron and His Sons
Yalkut Shimoni on Torah 485:1
(ויקרא ו ט) וְהַנּוֹתֶרֶת מִמֶּנָּה, כְּשֵׁרָה וְלֹא פְּסוּלָה, יֹאכְלוּ אַהֲרֹן וּבָנָיו, רִבָּה אֲכִילָתָהּ בְּכָל מַאֲכָל שֶׁיִּרְצוּ. יֹאכְלוּהָ, שֶׁיֹּאכְלוּ עִמָּהּ חֻלִּין וּתְרוּמָה בִּזְמַן שֶׁהִיא מוּעֶטֶת. לְאַהֲרֹן וּלְבָנָיו, לְאַהֲרֹן תְּחִלְּה וְאַחַר כָּךְ לְבָנָיו. לְאַהֲרֹן שֶׁלֹּא בְּמַחֲלֹקֶת וּלְבָנָיו בְּמַחֲלֹקֶת, מָה אַהֲרֹן כֹּהֵן גָּדוֹל יֹאכַל שֶׁלֹּא בְּמַחֲלֹקֶת, אַף בָּנָיו כֹּהֲנִים גְּדוֹלִים יֹאכְלוּ שֶׁלֹּא בְּמַחֲלֹקֶת. מַצוֹת תֵּאָכֵל, מִצְוָה. לְפִי שֶׁהָיְתָה בִּכְלַל הֶתֵּר וְנֶאֶסְרָה וְחָזְרָה וְהֻתְּרָה, יָכוֹל תַּחֲזֹר לְהֶתֵּרָהּ הָרִאשׁוֹן, תַּלְמוּד לוֹמַר "מַצּוֹת תֵּאָכֵל", מִצְוָה. כַּיּוֹצֵא בָּהּ "יְבָמָהּ יָבֹא עָלֶיהָ", מִצְוָה. לְפִי שֶׁהָיְתָה בִּכְלַל הֶתֵּר וְנֶאֶסְרָה וְחָזְרָה וְהֻתְּרָה, יָכוֹל תַּחֲזֹר לְהֶתֵּרָהּ הָרִאשׁוֹן, תַּלְמוּד לוֹמַר "יְבָמָהּ יָבֹא עָלֶיהָ", מִצְוָה. מָאן תַּנָא לְהָא דְּתָנוּ רַבָּנָן "יְבָמָה יָבֹא עָלֶיהָ" מִצְוָה וְכוּ'. אָמַר רַבִּי יִצְחָק בַּר אַבְדִּימִי, אַבָּא שָׁאוּל הִיא, וְהָכֵי קָאָמַר "יְבָמָהּ יָבֹא עָלֶיהָ" לְמִצְוָה, שֶׁבִּתְחִלָּה הָיְתָה עָלָיו בִּכְלַל הֶתֵּר, רָצָה לְשֵׁם נוֹי כּוֹנְסָהּ, רָצָה לְשֵׁם אִשּׁוּת כּוֹנְסָהּ, נֶאֶסְרָה וְחָזְרָה וְהוּתְרָה יָכוֹל תַּחַזוֹר לְהֵיתֵרָה הָרִאשׁוֹן, תַּלְמוּד לוֹמַר "יְבָמָהּ יָבֹא עָלֶיהָ" לְמִצְוָה. [רָבָא אָמַר, אֲפִלּוּ תֵּימָא רַבָּנָן, וְהָכֵי קָאָמַר "יְבָמָהּ יָבֹא עָלֶיהָ" לְמִצְוָה], שֶׁבִּתְחִלָּה הָיְתָה עָלָיו בִּכְלַל הֶתֵּר, רָצָה כּוֹנְסָהּ רָצָה (חוֹלֵץ לָהּ) [אֵינוֹ כּוֹנְסָהּ], וְנֶאֶסְרָה וְחָזְרָה וְהֻתְּרָה, יָכוֹל תַּחֲזֹר לְהֶתֵּרָהּ הָרִאשׁוֹן רָצָה כּוֹנְסָהּ רָצָה אֵינּוֹ כּוֹנְסָהּ, [רָצָה אֵינוֹ כּוֹנְסָהּ], הָא אַגִּידָא בֵּיהּ בִּכְדִי תִּיפּוּק, אֶלָּא אֵימָא רָצָה כּוֹנְסָהּ רָצָה חוֹלֵץ, תַּלְמוּד לוֹמַר "יְבָמָהּ יָבֹא עָלֶיהָ" לְמִצְוָה. אֵימָא רֵישָׁא מַצּוֹת תֵּאָכֵל לְמִצְוָה, שֶׁבִּתְחִלָּה הָיְתָה עָלָיו בְּהֶתֵּר וְכוּ'. בִּשְׁלָמָא לְרָבָא דַּאֲמַר כְּרַבָּנָן הִיא, הָכִי קָאֲמַר, מַצּוֹת תֵּאָכֵל, מִצְוָה. שֶׁבִּתְחִלָּה הָיְתָה בִּכְלַל הֶתֵּר רָצָה אוֹכְלָהּ רָצָה אֵינוֹ אוֹכְלָהּ, נֶאֶסְרָה וְחָזְרָה וְהֻתְּרָה, יָכוֹל תַּחֲזֹר לְהֶתֵּרָהּ הָרִאשׁוֹן [רָצָה אוֹכְלָהּ, רָצָה אֵינוֹ אוֹכְלָהּ], רָצָה אֵינּוֹ אוֹכְלָהּ, וְהָכְתִיב (שמות כט, לג) "וְאָכְלוּ אֹתָם אֲשֶׁר כֻּפַּר בָּהֶם" מְלַמֵּד שֶׁהַכֹּהֲנִים אוֹכְלִין וּבְעָלִים מִתְכַפְּרִים, אֶלָּא רָצָה הוּא אוֹכְלָהּ רָצָה כֹּהֵן אַחֵר אוֹכְלָהּ, תַּלְמוּד לוֹמַר "מַצּוֹת תֵּאָכֵל", מִצְוָה. אֶלָּא לְרַב יִצְחָק בַּר אַבְדִּימִי דַּאֲמַר כֻּלָּהּ כְּאַבָּא שָׁאוּל הִיא. הָכָא מַאי תְּרֵי גַּוְנֵי אִיכָּא, רָצָה מַצָּה אוֹכְלָהּ רָצָה חָלוּט אוֹכְלָהּ. הַאי חָלוּט הֵיכִי דָמִי אִי מַצָּה הִיא הֲרֵי מַצָּה וְאִי לָאו מַצָּה הִיא, הֲרֵי מַצּוֹת אֲמַר רַחֲמָנָא. לְעוֹלָם מַצָּה הִיא וּלְהָכִי תְּנֵינָא רַחֲמָנָא, לְעַכֵּב.