Reference for Bekhorot 94:26

ולכתבו הרשאה להדדי

And should you say that the Mishnah [which says that he is not a first-born of inheritance] refers to a case where no power of attorney was given, is not [the Mishnah] explained later [in this chapter] as referring to a case where a power of attorney was written out, [thus proving that the power of attorney here does not help at all]? - [The Mishnah] supports the opinion of R'Jannai.

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