סימן קיז סעיף א
אִם נִמְצְאָה אֵיזוֹ תַעֲרֹבֶת חָמֵץ בְּעֶרֶב פֶּסַח, עַד הַלַּיְלָה, הֲרֵי הוּא כְּמוֹ שְׁאָר אִסּוּרִין שֶׁבְּטֵלִים בְּשִׁשִּׁים. וְלָכֵן אִם נִמְצָא גַּרְעִין בְּעוֹף וּבְתַבְשִׁיל, זוֹרְקוֹ, וְהַשְּׁאָר מֻתָּר לֶאֱכוֹל אֲפִלּוּ בְּפֶסַח. אֲבָל בְּתוֹךְ הַפֶּסַח חָמֵץ אוֹסֵר אֲפִלּוּ בְּמַשֶּׁהוּ גַם בַּהֲנָאָה. וּבְכָל מָקוֹם שֶׁנִּמְצָא אֵיזֶה גַּרְעִין מֵחֲמֵשֶׁת מִינֵי דָּגָן אוֹ מַשֶּׁהוּ חָמֵץ, צְרִיכִין לַעֲשׂוֹת שְׁאֵלָה.
If you find any chametz in food1This is said, in reference to chametz in liquid form, that has become mixed with other liquid; or if the chametz has contributed to the taste of the mixture. Regarding dry chametz, that has been mixed with other dry foods, (see Shulchan Aruch and Ramah 447:4.) on erev Pesach, anytime before nightfall, it is like any other forbidden substance, which is nullified if it becomes mixed (into a permitted subtance), sixty times its volume.2This is true also if the chametz and the other food are of the same species, i.e. vinegar and vinegar, soup and soup. (Biyur Halachah 447:2) Therefore, if you find a grain of seed in poultry or in cooked food, you should throw away the seed, and you may eat the rest, even on Pesach.3This is true, only when the cooked food had been cooled before Pesach. (If the food was still warm on Pesach, it had absorbed the chametz on Pesach and would be forbidden). (Mishnah Berurah 447:19) But during Pesach, itself, even the smallest bit of chametz makes food forbidden (to eat) and to derive benefit from it. Thus, wherever you find a seed of the five species of grain, or a small particle of chametz, you should consult a competent Posek.
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.)