מִצְוַת קָרְבַּן מוּסָף שֶׁל שַׁבָּת – שֶׁנִּצְטַוּוּ יִשְׂרָאֵל לְהַקְרִיב שְׁנֵי כְּבָשִׂים קָרְבָּן בְּכָל יוֹם שַׁבָּת, מוּסָף עַל קָרְבַּן הַתָּמִיד שֶׁל כָּל יוֹם, וְהוּא הַנִּקְרָא מוּסַף שַׁבָּת, שֶׁנֶּאֱמַר (במדבר כח ט) וּבְיוֹם הַשַּׁבָּת שְׁנֵי כְבָשִׂים וְגוֹ'. The commandment of the additional sacrifice of Shabbat: That Israel was commanded to sacrifice two lambs [as a] sacrifice on every Shabbat day, in addition to the regular sacrifice of every day. And it is called the addition of Shabbat, as it is stated (Numbers 28:9), “And on the Shabbat day, two lambs, etc.”
מִשָּׁרְשֵׁי מִצְוַת הַמּוּסָפִין שֶׁל יָמִים טוֹבִים כְּבָר כָּתַבְתִּי בְּסֵדֶר אֱמֹר אֶל הַכֹּהֲנִים בְּמוּסַף פֶּסַח (מצוה רצט) מַה שֶׁנִּרְאֶה לִי עַל צַד הַפְּשָׁט, וּמוּסָף שַׁבָּת בְּאֶשְׁכּוֹל יֶתֶר הַמּוּסָפִין תָּלוּי גַּם הוּא, וְהָעִנְיָן, כְּדֵי שֶׁנִּקְבַּע בְּמַחְשַׁבְתֵּנוּ עִם מַעֲשֵׂה הַקָּרְבָּן, גֹּדֶל הַיּוֹם וְרֹב קְדֻשָּׁתוֹ וְכִי שֵׁשֶׁת יָמִים עָשָׂה יְיָ אֶת הַשָּׁמַיִם וְאֶת הָאָרֶץ וְגוֹ'. כְּמוֹ שֶׁכָּתוּב שָׁם, וְשָׁם כָּתַבְתִּי גַּם כֵּן מְעַט מִדִּינֵי הַמּוּסָפִין וּכְלַל הָעִנְיָן, כְּמִנְהָגִי [שם]. I have already written from the roots of the commandment of the additional sacrifices of the holidays in the Order of Emor el HaKohanim regarding the addition of Pesach (Sefer HaChinukh 299) from the angle of the simple understanding. And the addition of Shabbat is also appurtenant to the cluster of additional sacrifices. And the matter is that with the act of the sacrifice, we fix in our thoughts the greatness of the day and its broad holiness, and that “in six days, the Lord made the heavens and the earth, etc.” — as it is written there. And there I also wrote a little of the laws of the additional sacrifices and the main principle of the matter, as is my custom.