Reference for Ketubot 169:2

פגע בהו רב פנחס בר אמי אמר להו רב ושמואל דאמרי תרוייהו והוא שצבורין ומונחין ברה"ר אמר להו אנן נמי מחריפותא דנהרא תפיסנא

They were met by R. Pinchas b. Ammi who said to them: Both Rav and Shumel ruled that “[Seizure is valid] only if [the produce] was piled up and lay in a public domain.” They replied to them, “We have seized it at the main current of the river.”

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