To not subtract from the commandments of the Torah: That we were prevented from subtracting a thing from that which the perfect Torah commanded us. And about this is it stated (Deuteronomy 13:1), “do not subtract from it.” And how is this prohibition? For example, that which they, may their memory be blessed, said in the third chapter of Rosh Hashanah 28b that if blood that requires one sprinkling is mixed up with blood that requires four sprinklings, “Rabbi Eliezer says, ‘It is to be sprinkled four times.’ And Rabbi Yehoshua says, ‘It is to be sprinkled once — since when you sprinkle four times, you transgress, “do not add,” and you perform an act; whereas when you sprinkle once, even if you transgress “do not subtract,” you do not perform an act.’” To here [is the excerpt from the Talmud]. We know from this that the negative commandment of “do not subtract” is in this way and all that is similar to it.
From the roots of the commandment is like the matter that we said in the commandment of “do not add,” that preceded it. And the rest of all of its matters are like it [as well].
שֶׁלֹּא לִגְרֹעַ מִמִּצְוֹת הַתּוֹרָה – שֶׁנִּמְנַעְנוּ שֶׁלֹּא לִגְרֹעַ דָּבָר מִמָּה שֶׁחִיְּבַתְנוּ תּוֹרָתֵנוּ הַשְּׁלֵמָה, וְעַל זֶה נֶאֱמַר (דברים יג א) וְלֹא תִגְרַע מִמֶּנּוּ. וְכֵיצַד יִהְיֶה זֶה הָאִסּוּר? כְּגוֹן מַה שֶׁאָמְרוּ זִכְרוֹנָם לִבְרָכָה בְּפֶרֶק שְׁלִישִׁי מֵרֹאשׁ הַשָּׁנָה (כח, ב) שֶׁאִם נִתְעָרֵב דָּם הַנִּתָּן בְּמַתָּנָה אַחַת עִם דָּם הַנִּתָּן בְּמַתַּן אַרְבַּע, רַבִּי אֱלִיעֶזֶר אוֹמֵר יִנָּתְנוּ בְּמַתַּן אַרְבַּע, רַבִּי יְהוֹשֻׁעַ אוֹמֵר יִנָּתְנוּ בְּמַתָּנָה אֶחָת, שֶׁאִם אַתָּה נוֹתֵן בְּמַתַּן אַרְבַּע עוֹבֵר עַל בַּל תּוֹסִיף וְעוֹשֶׂה מַעֲשֶׂה, וּכְשֶׁאַתָּה נוֹתֵן בְּמַתָּנָה אַחַת אַף עַל פִּי שֶׁאַתָּה עוֹבֵר עַל בַּל תִּגְרַע אִי אַתָּה עוֹשֶׂה מַעֲשֶׂה, עַד כָּאן. יָדַעְנוּ מִזֶּה, שֶׁבְּדֶרֶךְ זוֹ וְכָל כַּיּוֹצֵא בָּהּ, הוּא לָאו דְּלֹא תִגְרַע.
To not subtract from the commandments of the Torah: That we were prevented from subtracting a thing from that which the perfect Torah commanded us. And about this is it stated (Deuteronomy 13:1), “do not subtract from it.” And how is this prohibition? For example, that which they, may their memory be blessed, said in the third chapter of Rosh Hashanah 28b that if blood that requires one sprinkling is mixed up with blood that requires four sprinklings, “Rabbi Eliezer says, ‘It is to be sprinkled four times.’ And Rabbi Yehoshua says, ‘It is to be sprinkled once — since when you sprinkle four times, you transgress, “do not add,” and you perform an act; whereas when you sprinkle once, even if you transgress “do not subtract,” you do not perform an act.’” To here [is the excerpt from the Talmud]. We know from this that the negative commandment of “do not subtract” is in this way and all that is similar to it.
מִשָּׁרְשֵׁי הַמִּצְוָה. כְּעִנְיָן שֶׁאָמַרְנוּ בְּמִצְוֹת בַּל תּוֹסִיף הַקּוֹדֶמֶת לָהּ, וְיֶתֶר כָּל עִנְיָנֶיהָ, כְּמוֹתָהּ.
From the roots of the commandment is like the matter that we said in the commandment of “do not add,” that preceded it. And the rest of all of its matters are like it [as well].