Genesis 17:17 then abraham fell upon his face, and laughed, and said in his heart, shall a child be born unto him that is a hundred years old? and shall sarah, that is ninety years old, bear?
The Old Testament character whose name means laughter is Isaac. His name was given to him because his mother Sarah laughed when she heard she would have a son in her old age.
Abraham loved both Sarah and Hagar, but in different ways. Sarah was his wife and the mother of his son Isaac, who was the child of the promise. Hagar was his concubine and the mother of his son Ishmael, who was born before the promise of a son to Abraham and Sarah.
God told Abraham that he would have a son at his old age.
Ishmael and his mother, Hagar, were thrown out into the wilderness by Abraham due to Sarah's jealousy and demands. Sarah, Abraham's wife and Ishmael's stepmother, felt threatened by Ishmael's presence as he was also Abraham's son, prompting Abraham to send them away as he believed it was God's will.
Abraham laughed in happy wonderment that he and Sarah, both aged, would soon have a son (Genesis 17).See also the Related Link.More about Abraham
Abraham did not believe what God told him. God told Abraham that he and his wife Sarah would have a baby. But Abraham thought it was impossible because Sarah was in her 90s. So Sarah told Abraham to sleep with with Hagar there servant, there child was Ishmael. Later, Sarah changed her mind and made Abraham send Hagar and Ishmael away. Some time after this Abraham slept with Sarah and she became pregnant with Isaac.
God promised Abraham and Sarah that they would have many descendants and that they would be the ancestors of a great nation. He also promised them the land of Canaan as an everlasting possession for their descendants.
In the Bible, God did not explicitly tell Abraham to listen to his wife Sarah. However, God did promise to fulfill His covenant through Sarah's child, Isaac. Ultimately, Abraham upheld his end of the bargain with Sarah, demonstrating trust in God's plan.
Sarah tried to make God's promise to Abraham come to pass, by having him take her maidservant as a second wife, as per the custom in those days. The promise was that Abraham would have descendants.See also:More about Abraham
I believe the short answer is unbelief. God has promised a son to Abraham, and that this son would come through Sarah, even though Sarah was advanced in years. Either Sarah didn't believe God at all, or she didn't believe the promised child had to come through her. Isaac was the son born of the promise, through Sarah as God had promised. The first comment is quite correct. It has also been discovered that this was an ancient practice and so her actions were not without precedent even though prompted by unbelief. Discovery of this ancient custom underlines the social 'fitness' of the Genesis record of history as opposed to the Documentary view which puts the composition much later when such social customs would have been unknown.
Ishmael was driven out because Sarah, Abraham's wife, was jealous of him being the son of Hagar, her maid. Sarah asked Abraham to send Hagar and Ishmael away to ensure that Isaac, her own son, would be the sole heir.
Initially, their relationship is probably alright. Hagar is one of Sarah's slaves, and Sarah choose her to be a mistress for Abraham. I kind of understand the legal implications of this. In a worldly sense, the important thing was that Abraham have an heir. Because of Sarah's lack of faith in God to do as he had promised, or perhaps considering that she wasn't the one who would bear Abraham the promised son, she decided Hagar could bear a child for Abraham in her name. As soon as Hagar conceives however, animosity develops between the two women, as Hagar "despises" Sarah. (Genesis 16:4) There must be some cultural implications that I'm missing here, because I don't really understand why Hagar would despise Sarah... perhaps because her son wouldn't be her own, but would be Sarah's son by law? An interesting question that is definitely worth looking into.