Yevamot 62B

בני בנים הרי הן כבנים! כי תניא ההיא, להשלים.

Grandchildren are considered like children. This indicates that if one’s children have passed away, he has fulfilled the mitzva to be fruitful and multiply only if they had children of their own, as they are considered like his own children. The Talmud responds: When that baraita is taught it is with regard to completing the required number of children, e.g., if he had only a son, but his son had a daughter, he has fulfilled the mitzva to be fruitful and multiply.

מיתיבי: בני בנים הרי הם כבנים, מת אחד מהם או שנמצא סריס ⁠— לא קיים פריה ורביה. תיובתא דרב הונא תיובתא.

The Talmud raises an objection to the opinion of Rav Huna from another baraita: Grandchildren are considered like children. If one of a man’s children died or was discovered to be a eunuch, the father has not fulfilled the mitzva to be fruitful and multiply. This directly contradicts Rav Huna’s statement that one fulfills the mitzva even if his children die. The Talmud concludes: The refutation of the opinion of Rav Huna is indeed a conclusive refutation.

בני בנים הרי הם כבנים. סבר אביי למימר, ברא לברא וברתא לברתא, וכל שכן ברא לברתא. אבל ברתא לברא ⁠— לא. אמר ליה רבא: ״לשבת יצרה״ בעיא, והא איכא.

§ It was taught in the baraita that grandchildren are considered like children. Abaye thought to say that if one’s children die, he fulfills the mitzva to be fruitful and multiply through grandchildren, provided a son was born to his son and a daughter to his daughter, and all the more so if a son was born to his daughter, as his grandchildren take the place of his children in these cases. However, if a daughter was born to his son, no, she cannot take the place of her father. Rava said to him: We require merely fulfillment of the verse: “He formed it to be inhabited,” and there is fulfillment in this case, as the earth is inhabited by his descendants.