Reference for Gittin 30:5

אלא או כדרבא או כדרב אשי

R. Ashi strongly demurred to this [reasoning]. Is there anything. he said, which if stated by one persons is valid, but becomes invalid if another joins with him? No, said R. Ashi, what we have to say is that even if the bearer Says. 'I myself am the second witness', [the Get] is invalid, because in regard to both signatures alike we must either insist on confirmation or follow the regulation of the Rabbis.

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