Devarim 13:1
Deuteronomy 13:1
"Everything I command you, you shall observe to do; you shall not add to it nor subtract from it" — the Torah's prohibition against innovation in religious practice (bal tosif).
אֵ֣ת כׇּל־הַדָּבָ֗ר אֲשֶׁ֤ר אָנֹכִי֙ מְצַוֶּ֣ה אֶתְכֶ֔ם אֹת֥וֹ תִשְׁמְר֖וּ לַעֲשׂ֑וֹת לֹא־תֹסֵ֣ף עָלָ֔יו וְלֹ֥א תִגְרַ֖ע מִמֶּֽנּוּ׃ {פ}
Be careful to observe only that which I enjoin upon you: neither add to it nor take away from it.
Why it matters — The Gra's fundamental objection was that Chassidim were introducing dangerous innovations — changing the nusach (prayer rite) from Ashkenaz to Sefard, altering the times of prayer, and creating new religious structures — violating this biblical principle of preserving received tradition.