By then Sarai was very old, but God said she would bring forth her own son, and he changed her name to Sarah. Sarah laughed at the very idea, but she did have a baby, and he was called Isaac. He was the father of Jacob, and his descendants became the 12 tribes of Jews. Already before the birth of Isaac, trouble had begun. Hagar despised Sarai for being a barren old hag.
Nobody appreciates that kind of remark.
When Isaac was weaned, and Sarah loved him more than his brother, she decided that Ishmael would not become Abraham's heir. Abraham wanted to keep the boy, but Sarah said Hagar and her son Ishmael should get lost, which they did. And the family quarrel between the Jewish cousins and the Arab cousins has continued ever since. Abraham felt so concerned about all that was going wrong that he felt God was calling him to make a supreme sacrifice. So he took his son to Mount Moriah, which is now called Jerusalem.