Reference for Chullin 149:44

טעמא דידי נמי מהכא

but according to you, why should it not be offered [upon the altar]? - He replied: It is like an animal which has not reached the prescribed age.<span class="x" onmousemove="('comment',' Which in the first seven days of its life, though in every respect an animal, may not be offered as a sacrifice (cf. Ex. XXII, 29) . Likewise with the fat of the foetus, although it is regarded as fat in every respect, it is nevertheless forbidden for sacrificial purposes.');">²⁶</span>

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