Reference for Bava Kamma 72:5

וכן כי אתא רבין אמר ר' יוחנן משום פשיעת שומרין נגעו בה

So too, when Rabin returned [from Eretz Yisrael] he stated on behalf of R. Johanan that it was for the failure [to carry out their duty] as bailees that liability was incurred [by the earlier plaintiffs to the later].

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