Sotah9: 4
מֵאַיִן הָיוּ מוֹדְדִין? רַבִּי אֱלִיעֶזֶר אוֹמֵר: מִטִּבּוּרוֹ. רַבִּי עֲקִיבָא אוֹמֵר: מֵחָטְמוֹ. רַבִּי אֱלִיעֶזֶר בֶּן יַעֲקֹב אוֹמֵר: מִמְּקוֹם שֶׁנַּעֲשָׂה חָלָל — מִצַּוָּארוֹ.
From where would they measure? R’ Elizer says: From his navel. R’ Akiva says: From his nose. R’ Elizer ben Yaakov says: From the place that he became a corpse —- his neck.