Reference for Bekhorot 101:44

כ"מ שאתה מוצא שתי כללות הסמוכים זה לזה הטל פרט ביניהם ודונם בכלל ופרט:

As therefore the specification explicitly mentions a movable object and that which is itself money, so everything [with which we may redeem] must be a movable object and that which is itself money.

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