Reference for Bekhorot 116:8

כונסן לדיר ועושה להם פתח קטן כדי שלא יהו שנים יכולין לצאת כאחת ומונין

- You might have said that we ought to enact a prohibition lest he should come to take from these.<span class="x" onmousemove="('comment',' Born in Ab and Tishri, and this would lead to an offence against the law by bringing hullin to the Temple, as, for example, if he takes one of the animals born in Ab as the tithe for the others. If Elul is the New Year, it will be found that these cannot combine for tithing and consequently the one taken is not the tithe, and if therefore he eats this as the tithe he will be eating hullin in the Temple. The same applies if he took one lamb from those born in Tishri.');">⁸</span>

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