Reference for Sanhedrin 4:2
דִּינֵי הַטֻּמְאוֹת וְהַטָּהֳרוֹת מַתְחִילִין מִן הַגָּדוֹל, דִּינֵי נְפָשׁוֹת מַתְחִילִין מִן הַצָּד. הַכֹּל כְּשֵׁרִין לָדוּן דִּינֵי מָמוֹנוֹת וְאֵין הַכֹּל כְּשֵׁרִין לָדוּן דִּינֵי נְפָשׁוֹת, אֶלָּא כֹהֲנִים, לְוִיִּם, וְיִשְׂרְאֵלִים הַמַּשִּׂיאִין לַכְּהֻנָּה:
Cases of (ritual) uncleanliness and cleanliness begin from the senior (judge). Capital cases begin from the side [i.e., from those lesser in wisdom, who sat at the side, it being written (Exodus 23:2): "Do not answer upon riv (written "rav") to deviate." That is, do not answer after the elect one in beth-din, to deviate from his words. For this reason, his words are heard only at the end.] All are kasher to adjudicate monetary litigations, [even a proselyte, if his mother were a Jewess. And a mamzer, too, is kasher to adjudicate monetary litigations.] But not all are kasher to adjudicate capital cases, but only Cohanim, Levites, and Israelites who can marry into the priesthood, [it being written (Exodus 18:22): "And let them lighten your burden and bear with you (Moses)" — they must be like you. Just as Moses our teacher was "pedigreed," so beth-din must be pedigreed.]