Midrash for Chullin 70:20
הוכשרו בשחיטה:
R'Assi said that R'Simeon was of the opinion that only the slaughtering renders an animal susceptible to uncleanness but not the blood.<span class="x" onmousemove="('comment',' I.e., the blood of a slaughtered animal will in no circumstances render any foodstuff susceptible to uncleanness.');"><sup>9</sup></span>
Sifrei Devarim
"On the earth shall you spill it as water": Just as it is permitted to derive benefit from water, it is permitted to derive benefit from blood — Just as water renders seed susceptible (of incurring tumah), so, blood. Just as water need not be covered, so blood (that of a disqualified offering which has been slaughtered) need not be covered.
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