Reference for Bekhorot 106:29

ונוהג בבקר ובצאן ואין מתעשרין מזה על זה בכבשים ובעזים וכו':

is because the Divine Law says: 'Shall be holy', but otherwise the holiness of an animal set aside for tithe would have applied to consecrated animals.<span class="x" onmousemove="('comment',' Applying to it the prohibition attached to an animal tithed concerning which (as is explained later) Scripture says, 'It shall not be sold, It shall not be redeemed'.');">²¹</span>

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