(Exodus 12:10) "And you shall let none of it remain until morning." This teaches that if one did leave any of it over, he has transgressed a commandment. But I might still understand that what is left over is fit to use; therefore the verse states, "And you shall let none of it remain until morning" [it is forbidden]. I might read only the verse, "And that which remains of it you shall burn with fire" (Exodus 12:10). Why then do I need the words "until morning"? Rather, Scripture comes to set a boundary for the morning of the morning [to fix the precise time after daybreak when the prohibition takes effect].
Why Scripture Sets Daybreak as the Limit for the Leftover Passover Offering
Yalkut Shimoni on Torah 198:4
(שמות יב י) וְלֹא תוֹתִירוּ מִמֶּנּוּ עַד בֹּקֶר, הָא אִם הוֹתִיר עָבַר עַל מִצְוָה, אֲבָל שׁוֹמֵעַ אֲנִי יְהֵא כָּשֵׁר תַּלְמוּד לוֹמַר וְלֹא תוֹתִירוּ מִמֶּנּוּ עַד בֹּקֶר, אֲנִי אֶקְרָא וְהַנֹּתָר מִמֶּנּוּ בָּאֵשׁ תִּשְׂרֹפוּ, עַד בֹּקֶר לָמָּה לִי, אֶלָּא בָּא הַכָּתוּב לִתֵּן תְּחוּם לְבָקְרוֹ שֶׁל בֹּקֶר.