Reference for Rosh Hashanah 51:1

מקצתן נעשו עדים ומקצתן נעשו דיינין דברי רבי טרפון רבי עקיבא אומר כולן נעשין עדים ואין עד נעשה דיין

some of them act as witnesses and some as judges. This is the view of R'Tarfon. R'Akiba says: They all act as witnesses, and a witness cannot act as a judge'? - You may say that our Mishnah agrees even with R'Akiba.

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