Reference for Bekhorot 60:38

הבא לקבל דברי חבירות צריך לקבל בפני ג' חבירים ואפילו תלמיד חכם צריך לקבל בפני שלשה חבירים

An elder, a member of a scholars' council, is not required to accept [these obligations] in the presence of three haberim, having already accepted them from the time when he took his place at the council.

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