"And if the household is too few to be for a lamb" (Exodus 12:4). The verse comes to teach you that people may always be enrolled upon the Passover lamb and may withdraw from it until it is slaughtered, provided that one does not leave the Passover lamb as it is [with no one enrolled]. Rabbi Yehudah says: provided that there remained there one member of the first group, so as not to make the primary group secondary and the secondary group primary. "And they may withdraw from it until it is slaughtered." Rabbi Shimon says: until the blood is dashed. The Rabbis hold: "from being" [mihyot] means from the living state of the lamb; and Rabbi Shimon holds it means from the being of the lamb [i.e., once dashed]. But as for enrolling upon it, all agree it is until it is slaughtered, for Scripture says, "according to the number of souls" and afterward "you shall apportion it" (Exodus 12:4). It was taught: "too few to be for a lamb"—measured by what is needed for eating and not by what is needed for purchase. Rabbi says: also by what is needed for purchase, for if one has not enough, he enrolls others with him, and the money in his hand remains unconsecrated, for it was on this condition that Israel consecrated their Passover offerings. Rabbah and Rabbi Zeira: regarding the wood for its roasting all agree it is considered like its body; where they differ is regarding the unleavened bread and bitter herbs. The Rabbis hold these are a separate eating, while Rabbi holds that since they are part of what makes the Passover fit, they are like the Passover itself. And one said: regarding the unleavened bread and bitter herbs they also do not differ; where they differ is regarding using the money to buy a garment or a cloak, for the Rabbis hold "from being a lamb" means keep the lamb alive [count for the lamb's sake], while Rabbi holds it means keep yourself alive by means of the lamb.
If the Household Is Too Small for a Lamb
Yalkut Shimoni on Torah 192:4
וְאִם יִמְעַט הַבַּיִת מִהְיוֹת מִשֶּׂה, בָּא הַכָּתוּב לְלַמֶּדְךָ שֶׁלְּעוֹלָם נִמְנִין עַל הַפֶּסַח וּמוֹשְׁכִין יְדֵיהֶן מִמֶּנוּ עַד שֶׁיִּשָּׁחֵט וּבִלְּבַד שֶׁלֹּא יַנִּיחַ אֶת הַפֶּסַח כְּמוֹת שֶׁהוּא רַבִּי יְהוּדָה אוֹמֵר, וּבִלְבַד שֶׁהָיָה שָׁם אֶחָד מֵחֲבוּרָה רִאשׁוֹנָה שֶׁלֹּא לַעֲשׂוֹת עִקָר טְפֵלָה וּטְפֵלָה עִקָר. וּמוֹשְׁכִין אֶת יְדֵיהֶן מִמֶּנוּ עַד שֶׁיִּשָּׁחֵט רַבִּי שִׁמְעוֹן אוֹמֵר, עַד שֶׁיִּזָּרֵק הַדָּם וְרַבָּנָן סַבְרֵי, מִהְיוֹת, מֵחֲיוּתָא דְּשֶׂה. ור' שִׁמְעוֹן סָבַר, מֵהֲוָיָתָהּ דְּשֶׂה, אֲבָל לִמָּנוֹת עָלָיו דִּבְרֵי הַכֹּל עַד שֶׁיִּשָּׁחֵט, אָמַר קְרָא בְּמִכְסַת נְפָשֹׁת וַהֲדַר תָּכֹסּוֹ. תַּנְיָא מִהְיוֹת מִשֶּׂה מִכְּדֵי אֲכִילָה וְלֹא מִכְּדֵי מִקָּח, רַבִּי אוֹמֵר, אַף מִכְּדֵי מִקָּח, שֶׁאִם אֵין לוֹ מְמַנֶּה אֲחֵרִים עִמּוֹ וּמָעוֹת שֶׁבְּיָדוֹ חֻלִּין שֶׁעַל מְנַת כֵּן הִקְדִּישׁוּ יִשְׂרָאֵל אֶת פִּסְחֵיהֶן. רַבָּה וְר' זֵירָא בַּעֲצֵי צְלִיָּתוֹ כוּלֵי עַלְמָא לֹא פְּלִיגֵי דִּכְגוּפוֹ דָּמִי, כִּי פְּלִיגִי בְּמַצָּה וּמָרוֹר, רַבָּנָן סָבְרֵי, הָא אֲכִילָה אַחֲרִיתִי, וְרַבִּי סָבַר, כֵּיוַן דְּהֶכְשֵׁרוֹ דְּפֶסַח הוּא כְּפֶסַח דָּמִי. וְחַד אָמַר בְּמַצָּה וּמָרוֹר נַמִי לֹא פְּלִיגֵי כִּי פְּלִיגֵי לִקַּח בָּהֶן חָלוּק אוֹ טַלִּית, דְּרַבָּנַן סָבְרֵי, מִהְיוֹת מִשֶּׂה, הַחַיֵיהוּ לַשֶׂה, וְרַבִּי סָבַר, הַחֲיֵה עַצְמְךָ מִשֶׂה.