Halakhah for Yevamot 42:1
דאורייתא היא דכתיב (ויקרא יח, טו) ערות כלתך לא תגלה אימא כלת בנו וכלת בנו יש לה הפסק והא תניא כלתו ערוה כלת בנו שניה וכן אתה אומר בבנו ובן בנו עד סוף כל הדורות אלא אימא כלת בתו
Pentateuchal, it being written in the Scriptures, Thou shalt not uncover the nakedness of thy daughter-in-law?<span class="x" onmousemove="('comment',' Lev. XVIII, 15; why then did Rab include her among those of the second degree? ');"><sup>1</sup></span> — Read, 'the daughter-in-law of his son'. But is there any limitation<span class="x" onmousemove="('comment',' V. supra p. 125, n. 6. ');"><sup>2</sup></span> for the daughter-in-law of one's son? Surely it was taught: His daughter-in-law is a forbidden relative, and the daughter-in-law of his son is a forbidden relative of the second degree; and the same principle is to be applied to one's son and son's son to the end of all generations!<span class="x" onmousemove="('comment',' Ker. 14b. ');"><sup>3</sup></span>