Genesis 17:19
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. 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.
God promised that Sarah, Abraham's wife, would bear him a son indeed, and that his name would be called Isaac. With Isaac and with his seed and descendants after him God would establish His everlasting covenant, rather than with Ishmael. The promise was repeated in more definite and unmistakable terms to make clear that Sarah, not Hagar, would bear the covenant son. It corrected Abraham's assumption that Ishmael would be the heir through whom the promise of many nations would be fulfilled.
The promise was given when Abraham was nearly one hundred years old. He did not yet understand it but had turned in mind and affection to Ishmael, clinging to the belief that God's purposes would be accomplished through him. Ishmael was Abraham's beloved son, yet the promise of many nations and the covenant were not to be fulfilled through him. Though the covenant was not to be through Ishmael, God heard Abraham's prayer that Ishmael might live before Him and blessed Ishmael to make him a great nation. The assurance that Sarah would have a son was later repeated by angels.