Thank you for asking such a thought provoking question. I am not sure what scripture that you are referring to or what quotation of God that you have in mind here. Simply put, remember Ishmael was the son of Abraham by Sarah’s Egyptian slave girl Hagar. It was Isaac who the only son of Abraham by his wife Sarah. Hence, a vital link in the line of descent leading to Christ Since Sarah (Sarai) remained barren, she requested that Abraham have relations with Hagar, giving her to Abraham as his wife. But upon becoming pregnant, Hagar began to despise her mistress to such an extent that Sarah voiced complaint to her husband. “So Abram said to Sarai: ‘Look! Your maidservant is at your disposal. Do to her what is good in your eyes.’ Then Sarai began to humiliate her so that she ran away from her.” (Ge 16:1-6) At the fountain on the way to Shur, Jehovah’s angel found Hagar and instructed her to return to her mistress and to humble herself under her hand. Moreover, she was told that Jehovah would greatly multiply her seed and that the son to be born to her was to be called Ishmael. Abraham was 86 years old when Ishmael was born. Ge 16:7-16. The resultant difficulties made it apparent that this was not Jehovah’s way of fulfilling the promise previously made to Abraham concerning the “seed.” (Ge 15:1-16) Becoming aware of her pregnancy, Hagar began despising her mistress. When Sarah voiced complaint, Abraham granted his wife full authority to deal with Hagar as her maidservant. Humiliated by Sarah, Hagar ran away from her mistress but returned in obedience to divine direction, after which she gave birth to Ishmael. Ge 16:4-16. About 13 years after Ishmael’s birth, on the occasion of Abraham’s being divinely commanded to circumcise all the males of his household, Abraham was also instructed to call his wife, no longer by the name “Sarai,” but “Sarah,” meaning “Princess.” Regarding Sarah, God said: “I will bless her and also give you a son from her; and I will bless her and she shall become nations; kings of peoples will come from her.” (Ge 17:9-27) This scripture shows us the vital role that Isaac was to play. When informed that Sarah would also have a son from whom “kings of peoples” would come, Abraham petitioned God in behalf of his firstborn: “O that Ishmael might live before you!” God’s reply, after declaring that the future son Isaac would be the covenant heir, was: “As regards Ishmael I have heard you. Look! I will bless him and will make him fruitful and will multiply him very, very much. He will certainly produce twelve chieftains, and I will make him become a great nation.” (Ge 17:16, 18-20) At the age of 89 Ishmael assisted Isaac in burying their father Abraham. After that he lived another 48 years, dying in 1795 B.C.E. at the age of 137. (Ge 25:9, 10,17) There is no record of Ishmael’s being buried in the cave of Machpelah, the place of burial for Abraham and Isaac, along with their wives. Ge 49:29 31. In his dealings with Hagar and Ishmael, God clearly shows his concern and love for these ones, but clearly the favor is shown to Isaac for the role that he was to play in the lineage of God’s seed. Source: scriptures sighted NW Translation and Insight on the Scriptures, Volume I pages 1225, 1226

Perryman's answer is good; but let's simplify. Sarah was impatient for the promised child to be born and suggested that she would adopt the child of their Egyptian slave at birth. (Slaves are not consulted about whether they want to give up their children, or about anything else.) But the Egyptian was always only a concubine, that is, a consort without the legal rights of a wife, such as the right that her children would become heirs of the father--it was a way for poor women with nothing to offer but themselves to find husbands, even if they sometimes had to share them, and it was better than childlessness, or starving while trying to support a child born out of wedlock, or childlessness and eventual starvation in old age. Since Ishmael was not the legitimate heir, he didn't get the promise of the Messiah. The Koran probably has another take on this but my copy is in another building, sorry. (Not that I'm overly concerned with what it says, but should mention it for the sake of thoroughness.)

IMO this story is a rewritten version that doesn't make sense because of the rewriting. Isaac was important because he was the son of Sarah...being Abrahams son didn't matter. Which also is why the Jewish line is through the mother. They could rewrite certain things to lessen woman and make men more important but some things they couldn't change.

Roger If Hagar wife of Abraham was "only a concubine" why marry her at all ... after all she is only a slave girl do with her as you please? Before and after the Abrahamic era in all cultures the heir is the first son of the father not the mother. Ask a Jew are you your father or mother's son he will answer the father yet we have to square up the first born story and give the mother first place to explain ishmael away. The name should give you aq clue ending in el, the name of God known to Abraham