"Then God said, 'Yes, but your wife Sarah will bear you a son, and you will call him Isaac. I will establish my covenant with him as an everlasting covenant for his descendants after him. And as for Ishmael, I have heard you: I will surely bless him; I will make him fruitful and will greatly increase his numbers. He will be the father of twelve rulers, and I will make him into a great nation. But my covenant I will establish with Isaac, who Sarah will bear to you by this time next year.' When he had finished speaking with Abraham, God went up from him." NIV translation
God agreed to bless Ishmael meaning he would not have to worry about producing a son like Abram had worried.
Instead, God promised that he would be the father of twelve rulers and a great nation. This "great nation" speaks of the fact that they would be great in number but not great in pleasing God. This "great nation" was speaking of those who follow the teachings of Islam.
Although God agreed to bless Ishmael, He made it clear that the descendants of Isaac would be known as His people. (Ishmael would be the father of a great nation but not the father of the promised nation.) This was said twice in this passage and we must remember that, throughout the Bible, the number two is associated with truth as matters were established on the word of two witnesses.