Reference for Bekhorot 64:16

מי איכא דמחיים לא מיפריק ולאחר שחיטה מיפריק

For if when the animal is alive, when it is qualified to effect redemp tion [Scripture says that] it cannot be redeemed, after having been slaughtered, when it has not the strength to effect redemption, how much more so is it the case that it cannot be redeemed?

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