Reference for Berakhot 29:16

אמרי האי דקתני הקורא להודיעך כחו דר' יוסי דאמר דיעבד נמי לא דאי ר' יהודה אפי' לכתחלה נמי יצא

They answer that the fact that our Mishnah states Who reads is intended to show how extreme is the view of R. Jose, for he means to say that even post factum it is not valid; since were it R. Judah, even ab initio it would still be valid.

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