סימן פ סעיף ז

חַרְדָּל וְתִמְכָּא וּשְׁאָר מִינֵי טִבּוּלִין שֶׁלֹּא נָתַן בָּהֶם חֹמֶץ מִבְּעוֹד יוֹם, אָסוּר לִתֵּן בָּהֶם בַּשַׁבָּת, אֶלָּא בְּשִׁנּוּי, דְּהַיְנוּ, שֶׁיִּתֵּן תְּחִלָּה אֶת הַחֹמֶץ בַּכְּלִי, וְאַחַר כָּךְ יִתֵּן לְתוֹכוֹ אֶת הַחַרְדָּל אוֹ הַתִּמְכָּא. וְלֹא יַעֲשֶׂנוּ בְּלִילָה עָבָה, אֶלָּא יִשְׁפּוֹךְ הַרְבֵּה, שֶׁתְּהֵא בְּלִילָתוֹ רַכָּה, וְגַם לֹא יְעָרְבוֹ בְּכַף וְכַדּוֹמֶה, אֶלָּא בְּאֶצְבָּעוֹ אוֹ שֶׁיְנַעְנֵעַ בִּכְּלִי עַד שֶׁיִּתְעָרֵב (שכ"א).

It is forbidden to put vinegar into mustard, horseradish, and other relishes on Shabbos, in which no vinegar was put while it was still day (before Shabbos).²⁹Putting in liquid is prohibited because of losh (kneading), (one of the thirty-nine melachos). It is permitted if you do it in an irregular manner; that is, you must first put the vinegar into a dish, and afterwards put in the mustard or horseradish.³⁰If you put some liquid in before Shabbos, you may add more on Shabbos [Taz, Mishnah Berurah 321: 64]. A few drops, however, is insufficient for this purpose, there must be a noticeable quantity. (Magein Avraham, see Biur Halachah. You should not make a thick mixture, but you must pour enough vinegar into it so that the mixture is thin. You may not mix it with a spoon or anything similar, but with your finger, or you may shake the vessel until the contents are well mixed.³¹If you did put in the liquid on Shabbos, (bediavad), you may eat it [Biur Halachah]. This is because many poskim rule that the prohibition of losh applies only to kneading but not to pouring in liquid. (The opinion of the Semag, Semok, Seifer Haterumah and Yerei’im)

תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)