Reference for Beitzah 8:1

איכו השתא אשתלאי ואמרי לך רב ורבי יוחנן הלכה כרבי יוחנן הא אמר רבא הלכתא כותיה דרב בהני תלת בין לקולא בין לחומרא

"Oh that now, if you had asked me [yesterday], I [would have] said to you: [In a case where] Rav and Rabbi Yohanan [dispute halakhah], the halakhah is according to Rabbi Yohanan. But Rava said: 'The halakhah is according to Rav in these three cases, whether strict or lenient.'" [This is one of the three cases; the other two are explained later in the Gemara].

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