Reference for Avodah Zarah 117:2

ותיפוק ליה דהא מחוברין נינהו לא צריכא דתלשינהו גלא

But it should be excluded for the reason that it is something attached to the ground! No; it is necessary [to mention it because it can be prohibited in the case] where a wave detached it from the ground.

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