Reference for Bekhorot 116:3

ואי באחד בתשרי ר"ה דאב ואלול מצטרפין דתשרי פטורין

and if the first of Tishri is the New Year, the animals of Ab and Elul combine and those of Tishri are exempt.<span class="x" onmousemove="('comment',' As there are only five lambs and they belong to a different year.');">³</span>

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