Reference for Bekhorot 115:54

אמר רבא

As we have learnt: FIVE LAMBS BORN BEFORE ROSH HASHANAH AND FIVE LAMBS BORN AFTER ROSH HASHANAH DO NOT COMBINE [TO ENTER THE ONE SHED] WHEREAS FIVE LAMBS BORN BEFORE THE TITHING PERIOD AND FIVE AFTER THE TITHING PERIOD DO COMBINE.

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