Gen 17:1 And when Abram was ninety years old and nine, the LORD appeared to Abram, and said unto him, I [am] the Almighty God; walk before me, and be thou perfect. 2 And I will make my covenant between me and thee, and will multiply thee exceedingly.
7 And I will establish my covenant between me and thee and thy seed after thee in their generations for an everlasting covenant, to be a God unto thee, and to thy seed after thee. 8 And I will give unto thee, and to thy seed after thee, the land wherein thou art a stranger, all the land of Canaan, for an everlasting possession; and I will be their God.
Gen 17:15 And God said unto Abraham, As for Sarai thy wife, thou shalt not call her name Sarai, but Sarah [shall] her name [be].
16 And I will bless her, and give thee a son also of her: yea, I will bless her, and she shall be [a mother] of nations; kings of people shall be of her.
So the Lord allowed Sarah to bear a son.
17 Then Abraham fell upon his face, and laughed, and said in his heart, Shall [a child] be born unto him that is an hundred years old? and shall Sarah, that is ninety years old, bear?
18 And Abraham said unto God, O that Ishmael might live before thee!
19 And God said, Sarah thy wife shall bear thee a son indeed; and thou shalt call his name Isaac: and I will establish my covenant with him for an everlasting covenant, [and] with his seed after him.
So God established His covenant with the seed of Isaac.
20 And as for Ishmael, I have heard thee: Behold, I have blessed him, and will make him fruitful, and will multiply him exceedingly; twelve princes shall he beget, and I will make him a great nation. 21 But my covenant will I establish with Isaac, which Sarah shall bear unto thee at this set time in the next year. 22 And he left off talking with him, and God went up from Abraham.