Reference for Chullin 227:11

הפגול והנותר והטמא שבשלן בחלב חייב

or unclean [flesh] in milk, he is liable! - That Tanna is of the opinion that a prohibition can be superimposed upon an existing prohibition.<span class="x" onmousemove="('comment',' Whereas other Tannaim do not hold that view, and R. Ammi and R. Assi are in agreement with those other Tannaim.');">⁴</span>

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