Reference for Bekhorot 118:21

ירעו

Said Raba: A count properly begun redeems.<span class="x" onmousemove="('comment',' If he had ten lambs in the shed and he counted nine and the tenth died in the shed or passed through a different door from the others, the nine are redeemed and there is no need to combine them with the others of a later tithing period, since when he commenced counting the requisite number was available for tithing purposes.');">¹⁷</span>

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