Mishnayos Pesachim Perek 3 Mishnah 5

Dough at the beginning of the leavening process [siur], must be burned, but one who eats it is exempt from the punishment of karet because the dough had not become fully leavened. Dough that has reached the stage of cracking must be burned, and one who eats it intentionally is liable to receive karet, as he has intentionally eaten leavened bread during Passover. What is considered siur? Dough that has been leavened to the point that it has cracks that look like the antennae of locusts. The stage of cracking occurs later in the leavening process, when the cracks intermingle. This is the statement of Rabbi Yehuda. And the Rabbis say: One who intentionally eats either this or that, dough with cracks like locust antennae or with cracks that have become intermingled, is liable to receive karet, as once dough begins to crack it has certainly become leavened. And what is siur? It is any dough whose surface has becomes pale like the face of a person whose hair stands on end due to fear.

שִׂאוּר, יִשָּׂרֵף, וְהָאוֹכְלוֹ פָטוּר. סָדוּק, יִשָּׂרֵף, וְהָאוֹכְלוֹ חַיָּב כָּרֵת. אֵיזֶהוּ שִׂאוּר, כְּקַרְנֵי חֲגָבִים. סָדוּק, שֶׁנִּתְעָרְבוּ סְדָקָיו זֶה בָזֶה, דִּבְרֵי רַבִּי יְהוּדָה. וַחֲכָמִים אוֹמְרִים, זֶה וָזֶה, הָאוֹכְלוֹ חַיָּב כָּרֵת. וְאֵיזֶהוּ שִׂאוּר, כָּל שֶׁהִכְסִיפוּ פָנָיו כְּאָדָם שֶׁעָמְדוּ שַׂעֲרוֹתָיו: