Reference for Pesachim 51:1

ורבא אמר לך עד כאן לא קאמר ר' יהודה שאין מתכוין כמתכוין אלא לחומרא אבל מתכוין כשאין מתכוין לקולא לא

Raba says thus: R'Judah rules that the unintentional is the same as the intentional only in the direction of stringency, but he did not rule that the intentional is the same as the unintentional where it is in the directi of leniency. Abaye said: Whence do I know it?

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