Reference for Pesachim 3:5
שְׂאֹר, יִשָּׂרֵף, וְהָאוֹכְלוֹ פָטוּר. סָדוּק, יִשָּׂרֵף, וְהָאוֹכְלוֹ חַיָּב כָּרֵת. אֵיזֶהוּ שְׂאֹר, כְּקַרְנֵי חֲגָבִים. סָדוּק, שֶׁנִּתְעָרְבוּ סְדָקָיו זֶה בָזֶה, דִּבְרֵי רַבִּי יְהוּדָה. וַחֲכָמִים אוֹמְרִים, זֶה וָזֶה, הָאוֹכְלוֹ חַיָּב כָּרֵת. וְאֵיזֶהוּ שְׂאֹר, כָּל שֶׁהִכְסִיפוּ פָנָיו כְּאָדָם שֶׁעָמְדוּ שַׂעֲרוֹתָיו:
Siur [partially fermented dough] must be burned, and one who eats it is not liable. Sidduk [When dough becomes chametz, it develops sedakim (slits)] must be burned, and one who eats it incurs kareth. Which is "siur"? Like the horns of hoppers [a slit here, a slit there]. "Sidduk" — intersecting slits. These are the word of R. Yehudah. The sages say: One who eats either incurs kareth, ["like the horns of hoppers" also being "sidduk." ] Which is "siur" [for which one is not liable]? That [which has no slits, but] the surface of which is ashen, as one whose hair stands on end [from fright, his face turning ashen. The halachah is in accordance with the sages.]