אורח חיים תס״דOrach Chayim 464

Se’if 1א׳

חַרְדָּל אוֹ שְׁאָר מִינֵי טִבּוּל שֶׁיֵּשׁ בָּהֶם מַיִם, אֵין נוֹתְנִין בָּהֶם קֶמַח; וְאִם נָתַן לְתוֹךְ הַחַרְדָּל, יֹאכַל מִיָּד וְלֹא יַשְׁהֶנּוּ שֶׁמָּא יַחְמִיץ, וּבִשְׁאָר מִינֵי טִבּוּלִין כֵּיוָן שֶׁאֵינָם חַדִּין כְּמוֹ חַרְדָּל, וַדַּאי הֶחְמִיץ וְצָרִיךְ לְשָׂרְפוֹ אִם הוּא אַחַר זְמַן אִסוּר חָמֵץ. הַגָּה: וְהַמִּנְהָג שֶׁלֹּא לֶאֱכֹל חַרְדָּל כְּלָל בְּפֶסַח, אֲפִלּוּ נִתְעָרֵב קֹדֶם פֶּסַח, דְּהָוֵי כְּמִינֵי קִטְנִיּוֹת שֶׁנּוֹהֲגִין בּוֹ אִסּוּר (הַגָּהוֹת מַיְמוֹנִי רפ"ה וְת"ה סִימָן קי"ג).

Laws of mustard into which flour has been mixed. Contains 1 section: Mustard or other types of condiments that contain water—one may not put flour into them. And if one placed [flour] into the mustard, it must be eaten immediately and should not be left to stand, lest it become leavened. And with other types of condiments, since they are not as sharp as mustard, it has certainly become leavened and must be burned if it is after the time when chametz is forbidden. Rema: And the custom is not to eat mustard at all on Passover, even if it was mixed before Passover, for it is considered like types of legumes which are customarily forbidden.