Reference for Yevamot 155:5

ואם נאמר דורות ולא נאמר בנים הייתי אומר לאותן העומדים על הר סיני לכך נאמר בנים

it might have been assumed that only the first and second son is forbidden but that the third<span class="x" onmousemove="('comment',' Though the son of a proselyte of the first generation. ');">¹⁶</span> is permitted, the expression of 'generations'<span class="x" onmousemove="('comment',' Indicating all the sons of the same generation. ');">¹⁷</span> was, therefore, used. And had the expression of 'generations' only been used and not that of sons,<span class="x" onmousemove="('comment',' Reading 'generations that are born'. ');">¹⁸</span> it might have been assumed that the precept was given only to those who stood at Mount Sinai,<span class="x" onmousemove="('comment',' And that Egyptians born three generations later than the date of the promulgation of the Law shall no more be subject to its restrictions. ');">¹⁹</span>

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