Reference for Makkot 46:15

אמר רבא ממאי דלמא יהודה כיון דחשיב ירחי ויומי ואיתרמי דחזינן מחזקינן דלא חזינן לא מחזקינן

Rava said, how do we know [that it was a heavenly voice]? Maybe by Yehuda, once he counted the days and nights and reasoned that we can make a presumption from what we know but we can not make a presumption from what we do not know.

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