Reference for Yevamot 72:9

א"ל אביי ממאי דלמא לא היא עד כאן לא קאמר ר' אליעזר הכא אלא משום דקפגע באיסור אשת אח דאורייתא אבל התם דרבנן כרבנן סבירא ליה

R. Eliezer's ruling might extend to the present case<span class="x" onmousemove="('comment',' Lit., 'R. Eliezer did not so far say (his ruling) here'. ');">²⁵</span> only because the levir is encroaching<span class="x" onmousemove="('comment',' It being possible that the child would be viable. ');">²⁶</span> upon the prohibition of 'brother's wife', which is Pentateuchal,<span class="x" onmousemove="('comment',' For such a serious offence a penalty is rightly imposed. ');">²⁷</span>

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