Midrash su Esodo 18:22

(Ib. b) "Neither should one be investigated beyond the investigation which qualified one to the Sanhedrion." What is the reason? R. Joseph was taught that just as the judges ought to be pure in their perfect conduct so also ought they to be pure in the blemish of their family relations. Said M'remar: "What is the passage to prove this? (Songs 4, 7) Thou art altogether beautiful, my beloved, and there is no blemish on thee." But perhaps this blemish means literally a bodily blemish, but not as to the genealogy? Said R. Acha b. Jacob: "The following passage will explain it (Num. 11, 16) And they shall stand there with thee; i.e.; with thee [just as Moses was perfect in every thing, so also were the seventy who qualified for the Sanhedrion]." But perhaps this is because of the Shechina. Said R. Nachman: "There is another passage (Ex. 18, 22) When they shall bear thee; i.e., in comparison with thee."