So Bathsheba went to King Solomon to speak to him about Adonijah. The king stood up to greet her, bowed to her, sat down on his throne, and had a throne placed for the king's mother. So she sat down at his right hand.
Then she said, "I have just one small request of you. Don't turn me down." "[Go ahead and] ask, mother," the king replied, "for I won't turn you down."
So she said, "Let Abishag the Shunammite be given to your brother Adonijah as a wife."
King Solomon answered his mother, "Why are you requesting Abishag the Shunammite for Adonijah? Since he is my elder brother, you might as well ask the kingship for him, for Abiathar the priest, and for Joab son of Zeruiah."
References for 1 Kings 2:22
Then Solomon took an oath by the Lord: "May God punish me and do so severely if Adonijah has not made this request at the cost of his life.
And now, as the Lord lives, the One who established me, seated me on the throne of my father David, and made me a dynasty as He promised-I swear Adonijah will be put to death today!"
References for 1 Kings 2:24
Then King Solomon gave the order to Benaiah son of Jehoiada, who struck down Adonijah, and he died.