Reference for Chullin 266:54

בתר חיובא אזלינן וחיובא גבי ישראל הוא

In this respect the law of the shoulder and the two cheeks and the maw is more strict than the law of the first of the fleece.<span class="x" onmousemove="('comment',' For if an Israelite bought the portions of an animal from a priest from which the priestly dues are taken he is bound to give the dues; hence it is clear that the obligation rests upon the part of the animal bought.');">³⁴</span>

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