"it shall you observe (tishmor) to do": R. Eliezer b. Yaakov says: (The intent is) to assign a negative commandment to all that is mentioned in this section.
"you shall not add to it and you shall not detract from it": From here they ruled: If (the blood of offerings) which are subject to one application (of blood on the altar), became intermixed with (the blood of others) which are subject to one application, one application should be made (sufficing for both). If (the blood of) offerings which are subject to four applications became intermixed with (the blood of others) which are subject to four applications, four applications should be made (sufficing for both). If (the blood of offerings) became intermixed with (the blood of offerings) which are subject to one application — R. Eliezer said: Four applications should be made; R. Yehoshua said: One application should be made. R. Eliezer: But (according to you) he transgresses "you shall not detract!" R. Yehoshua: But (according to you) he transgresses "You shall not add!" R. Eliezer: "You shall not add" applies only when it is alone (and not intermixed with the blood of another which is subject to four). R. Yehoshua: "You shall not detract" applies only when it is alone (and not intermixed with the blood of another which is subject to one). And R. Yehoshua said further: By applying (four), you transgress "you shall not add" — by performing an action (of addition), whereas by not applying four, you have, (indeed) transgressed "you shall not detract," but you have not performed an action.
Variantly: "you shall not add to it": Whence is it derived that additions are not to be made to a lulav or to tzitzith? From "you shall not add to it." And whence is it derived that nothing is to be subtracted from them? From "and you shall not detract from it."
And whence is it derived that if a Cohein began the priestly blessing, he should not say: Since I have begun to bless, I shall say (Devarim 1:11) "May the L-rd, the G-d of your fathers, add to you, etc." From (Ibid. 13:1) "All the thing (davar), etc." — Even speech (["davar" can also be understood as "speech"]) shall not be added to it.
(דברים יג א) אֵת כָּל הַדָּבָר אֲשֶׁר אָנֹכִי מְצַוֶּה אֶתְכֶם, שֶׁתְּהֵא מִצְוָה קַלָּה חֲבִיבָה עָלֶיךָ כְּמִצְוָה חֲמוּרָה.
(Devarim 13:1) "The entire thing that I command you": That a "slight" commandment be as beloved by you as a "weighty" one.
אֹתוֹ תִּשְׁמְרוּ לַעֲשׂוֹת, רַבִּי אֱלִיעֶזֶר בֶּן יַעֲקֹב אוֹמֵר: לִתֵּן לֹא תַעֲשֶׂה עַל כָּל עֲשֵׂה הָאָמוּר בַּפָּרָשָׁה.
"it shall you observe (tishmor) to do": R. Eliezer b. Yaakov says: (The intent is) to assign a negative commandment to all that is mentioned in this section.
לֹא תוֹסֵף עָלָיו וְלֹא תִגְרַע מִמֶּנּוּ, מִכָּן אָמְרוּ (זבחים פ״ח מ״י): הַנִּתָּנִים בְּמַתָּנָה אַחַת שֶׁנִּתְעָרְבוּ בַּנִּתָּנִים בְּמַתָּנָה אַחַת יִנָּתְנוּ בְּמַתָּנָה אַחַת.
"you shall not add to it and you shall not detract from it": From here they ruled: If (the blood of offerings) which are subject to one application (of blood on the altar), became intermixed with (the blood of others) which are subject to one application, one application should be made (sufficing for both). If (the blood of) offerings which are subject to four applications became intermixed with (the blood of others) which are subject to four applications, four applications should be made (sufficing for both). If (the blood of offerings) became intermixed with (the blood of offerings) which are subject to one application — R. Eliezer said: Four applications should be made; R. Yehoshua said: One application should be made. R. Eliezer: But (according to you) he transgresses "you shall not detract!" R. Yehoshua: But (according to you) he transgresses "You shall not add!" R. Eliezer: "You shall not add" applies only when it is alone (and not intermixed with the blood of another which is subject to four). R. Yehoshua: "You shall not detract" applies only when it is alone (and not intermixed with the blood of another which is subject to one). And R. Yehoshua said further: By applying (four), you transgress "you shall not add" — by performing an action (of addition), whereas by not applying four, you have, (indeed) transgressed "you shall not detract," but you have not performed an action.
דָּבָר אַחֵר: לֹא תוֹסֵף עָלָיו, מִנַּיִן שֶׁאֵין מוֹסִיפִים עַל הַלּוּלָב וְעַל הַצִּיצִית? תַּלְמוּד לוֹמַר: לֹא תוֹסֵף עָלָיו. מִנַּיִן שֶׁאֵין פּוֹחֲתִים מֵהֶם? תַּלְמוּד לוֹמַר: וְלֹא תִגְרַע מִמֶּנּוּ.
Variantly: "you shall not add to it": Whence is it derived that additions are not to be made to a lulav or to tzitzith? From "you shall not add to it." And whence is it derived that nothing is to be subtracted from them? From "and you shall not detract from it."
מִנַּיִן שֶׁאִם פָּתַח לְבָרֵךְ בִּרְכַּת כֹּהֲנִים, לֹא יֹאמַר הוֹאִיל וּפָתַחְתִּי לְבָרֵךְ אוֹמַר (דברים א יא) ״ה׳ אֱלֹהֵי אֲבוֹתֵיכֶם יוֹסֵף עֲלֵיכֶם״? תַּלְמוּד לוֹמַר: הַדָּבָר – אֲפִלּוּ דָבָר לֹא תוֹסֵף עָלָיו.
And whence is it derived that if a Cohein began the priestly blessing, he should not say: Since I have begun to bless, I shall say (Devarim 1:11) "May the L-rd, the G-d of your fathers, add to you, etc." From (Ibid. 13:1) "All the thing (davar), etc." — Even speech (["davar" can also be understood as "speech"]) shall not be added to it.