Genesis 17:19 - Laugh but Isaac's no joke.

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.

This establishes with certainty of whom the heir of the everlasting covenant. Before this confirmation, Abraham already had a son named Ishmael with his second wife Hagar and he laughed at the thought of Sarah bearing a child at such an old age. This confirmation, however, clearly states that Sarah should bear the child and that the child's name should be called Isaac.

The importance of knowing that Sarah would bear the promised seed was exemplified when tree angels took time out before destroying Sodom and Gomorrah to confirm with Abraham that Sarah would bear a son, Genesis 18:10.