Reference for Bekhorot 119:35

רב כהנא אמר

Now there is no difficulty according to him who holds: The holiness of the tenth is determined by his counting; for this reason the ninth and the tenth are spoilt, and he calls the tenth the ninth and the eleventh the tenth.<span class="x" onmousemove="('comment',' There is some holiness in the eleventh, his naming it as the tenth having this effect.');">²⁸</span>

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