Reference for Chullin 163:29

אלא אמר רבי חייא בר אבא אמר רבי יוחנן

<big><b>MISHNAH: </b></big>IF TWO PERSONS BOUGHT A COW AND ITS YOUNG, HE WHO BOUGHT FIRST SHALL SLAUGHTER FIRST; BUT IF THE SECOND FORESTALLED HIM HE HOLDS HIS ADVANTAGE.

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