Midrash for Yevamot 124:1
בני בנים הרי הן כבנים כי תניא ההיא להשלים
Grandchildren are like children!<span class="x" onmousemove="('comment',' Infra 70a. It is now assumed that whenever one's own child died the grandchild may take his place in exempting his grandfather from the duty of propagation. From this it follows that only living children or grandchildren exempt a man from the duty of further propagation. How then could R. Huna maintain that dead children also exempt one from this duty? ');"><sup>1</sup></span> — This was taught only in respect of supplementing.<span class="x" onmousemove="('comment',' If a man had only one son he is exempt from the duty of propagation if his son had a daughter. If, however, he once had a male and a female who subsequently died he is in any case exempt. ');"><sup>2</sup></span> An objection was raised: Grandchildren are like children. If one of them died or was found to be a saris<span class="x" onmousemove="('comment',' V. Glos. ');"><sup>3</sup></span>
Kohelet Rabbah
“In the morning, sow your seed” – Rabbi Eliezer and Rabbi Yehoshua: Rabbi Eliezer says: If you sowed early in the season, sow [another field] late in the season, as you do not know which will endure for you, the early or the late. “As you do not know which will succeed, whether this or that.”
Rabbi Yehoshua says: If you married a woman in your youth and she died, marry in your old age. If you had children in your youth, you should have children in your old age, as it is stated: “In the morning, sow your seed, and in the evening do not rest your hand, as you do not know which will succeed, whether this or that.”
Rabbi Yishmael and Rabbi Akiva: Rabbi Yishmael says: If you studied Torah in your youth, study in your old age, as you do not know which Torah will endure, that of your youth or that of your old age, “or whether they both alike will be good.” Rabbi Akiva says: I had twelve thousand students from Gevat to Antipatris, and they all died during my lifetime, between Passover and Shavuot. Ultimately, seven promulgated my [Torah],35Seven students whom I taught in my old age were responsible for dissemination of my Torah. and these are they: Rabbi Yehuda, Rabbi Neḥemya, Rabbi Meir, Rabbi Yosei, Rabbi Shimon ben Yoḥai, Rabbi Eliezer son of Rabbi Yosei HaGelili, and Rabbi Yoḥanan the cobbler. He said to them: The first ones died only because they were begrudging to each other regarding their Torah. Do not be like that. They quickly rose and filled all of the Land of Israel with Torah.
Rabbi Natan