Commentary for Berakhot 29:15

וממאי דר' יוסי היא דילמא ר' יהודה היא ואמר גבי ק"ש נמי דיעבד אין לכתחלה לא תדע דקתני הקורא דיעבד אין לכתחלה לא

But how [do we know that the teaching in the quoted Mishnah] is R. Jose's? Perhaps it is R. Judah's, and he means to say with reference also to the reading of the Shema' that having read it inaudibly he has complied with the requirements of the law, but ab initio he should not read it so ? Thou mayest find proof for this in the fact that he states Who reads [the Shema' without making it audible], i.e. post factum it is valid, but not ab initio !

Abraham Cohen Footnotes to the English Translation of Masechet Berakhot