Why Scripture Says Abraham Sired Isaac and No Other Son

(Genesis 25:19:) THESE ARE THE GENERATIONS OF ISAAC, SON OF ABRAHAM. This is what Scripture says (Proverbs 17:6): "Grandchildren are the crown of the aged, and the glory of children is their fathers." What caused Abraham to be magnified? Jacob, as it is said (Isaiah 29:22): "Thus says the LORD to the house of Jacob, who redeemed Abraham." And why? Because if a person merits to be a son of Torah — he, and his son, and his son's son — it never again ceases from him forever, as it is said (Deuteronomy 4:10): "And you shall make them known to your children and to your children's children, the day on which you stood." Just as the day of the giving of the Torah is not annulled, so too one who teaches Torah to his son and to his son's son — it never again ceases from him. And so, when the Holy One, blessed be He, saw that Abraham was occupied with the Torah — from where? — as it is said (Genesis 26:5): "Because Abraham hearkened to My voice, etc.," and it is written (Genesis 18:19): "For I have known him." And Jacob arose and did not stir from the Torah, as it is said (Genesis 25:27): "And Jacob was a wholesome man, dwelling in tents." What did the Holy One, blessed be He, stipulate with them? (Joshua 1:8:) "This book of the Torah shall not depart, etc." And it is written (Proverbs 10:1): "A wise son gladdens a father" — this is Isaac, as it is said (Genesis 21:12): "For through Isaac shall seed be called to you." "But a foolish son" — this is Ishmael, as it is said (Genesis 25:12): "And these are the generations of Ishmael, son of Abram." There are sons who are disgraced through their fathers. Josiah was disgraced through his father, as it is said (II Chronicles 33:23): "For he, Amon, multiplied guilt." Hezekiah was disgraced through Ahaz, as it is said (Isaiah 8:16): "Bind up the testimony." And there are fathers who are disgraced through their sons. Eli was disgraced through his sons, [as it is said (I Samuel 2:12): "And the sons of Eli were worthless men," and Samuel too was disgraced through his sons], as it is said (I Samuel 8:3): "And his sons did not walk in his ways." But Abraham was not so; rather, he was magnified through Isaac, as it is said: "Abraham sired Isaac." And did he sire only Isaac alone? Is it not written [(Genesis 16:15): "And Hagar bore a son to Abram, etc."], and furthermore the sons of Keturah, Zimran and Jokshan? Yet it says only "Abraham sired Isaac" — because Isaac was righteous. Therefore it is said: "Abraham sired Isaac."