Reference for Ketubot 112:5

אמר אביי ודאי דדבריהם עבדו רבנן חיזוק ספק דדבריהם לא עבדו רבנן חיזוק רבא אמר בדמאי הקילו:

Abaye said: Any rabbinical enactment that is of a certain nature, the rabbis strengthened their words, but any rabbinical enactment of uncertain nature the rabbis did not strengthen their words. Rava said: They were lenient in respect of demai.

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