Reference for Berakhot 49:24

ולטעמיך אימא רישא כל זמן שמטפיחין הוא דאסור הא רשומן ניכר שרי אלא מהא ליכא למשמע מינה

But against thine own argument, I quote the first clause [of the Baraita brought in objection, viz. :] "With urine, so long as the ground is wet enough to moisten it is prohibited" ; hence if the trace of it be recognisable, it is permitted! No such deduction can be drawn. It is possible to say that [the question under consideration]

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