After the birth of Isaac the great joy manifested by Abraham and Sarah caused Hagar to be very jealous. Ishmael had been instructed by his mother that he was to be especially blessed of God, as the son of Abraham, and to be heir to that which was promised to him. Ishmael partook of his mother's feelings and was angry because of the joy manifested at the birth of Isaac. He despised Isaac, because he thought he was preferred before him. Sarah saw the disposition manifested by Ishmael against her son Isaac, and she was greatly moved. She related to Abraham the disrespectful conduct of Ishmael to her and to her son Isaac, and said to him, “Cast out this bondwoman and her son; for the son of this bondwoman shall not be heir with my son, even with Isaac.” SR 79.1
Abraham was greatly distressed. Ishmael was his son, beloved by him. How could he send him away? He prayed to God in his perplexity, for he knew not what course to take. The Lord informed Abraham, through His angels, to listen to the voice of Sarah his wife, and that he should not let his affections for his son or for Hagar prevent his compliance with her wishes. For this was the only course he could pursue to restore harmony and happiness again to his family. Abraham had the consoling promise from the angel, that Ishmael, although separated from his father's house, should not die nor be forsaken of God, that he should be preserved because he was the son of Abraham. God also promised to make of Ishmael a great nation. SR 79.2
Abraham was of a noble, benevolent disposition, which was manifested in his pleading so earnestly for the people of Sodom. His strong spirit suffered much. He was bowed with grief, and his paternal feelings were deeply moved as he sent away Hagar and his son Ishmael to wander as strangers in a strange land. SR 80.1
If God had sanctioned polygamy, He would not have thus directed Abraham to send away Hagar and her son. He would teach all a lesson in this, that the rights and happiness of the marriage relation are to be ever respected and guarded, even at a great sacrifice. Sarah was the first and only true wife of Abraham. She was entitled to rights, as a wife and mother, which no other could have in the family. She reverenced her husband, calling him lord, but she was jealous lest his affections should be divided with Hagar. God did not rebuke Sarah for the course she pursued. Abraham was reproved by the angels for distrusting God's power, which had led him to take Hagar as his wife and to think that through her the promise would be fulfilled. SR 80.2