Reference for Pesachim 211:2

רב אשי אמר טעמו פגמו וכוס של ברכה צריך שיעור חדא מילתא היא

R'Ashi said: [The deductions that] if he tastes it he renders it defective, and that the cup of Grace requires a [minimum] standard, are the same thing,<span class="x" onmousemove="('comment',' I.e., tasting it renders it unfit only when less than the minimum quantity is thereby left; otherwise it would remain fit.');">³</span> and this is what he Says: What is the reason that once he tastes of it he renders it defective?

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