Reference for Yevamot 83:4

אמר רבא ק"ו איסור כרת התרת איסור לאו לא כ"ש:

where a prohibition under the penalty of <i>kareth</i> is involved,<span class="x" onmousemove="('comment',' The marriage with the levir, where the widow gives birth to a viable child, is an act of incest which is punishable by kareth. ');">⁶</span> how much more so [should marriage be permitted<span class="x" onmousemove="('comment',' Three months after the death of the husband. ');">⁵</span>

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