Flavius Josephus, Antiquities of the Jews
William Whiston, A.M., Ed.

[3] But Adonijah, who, while his father was living, attempted to gain possession of the government, came to the king's mother Bathsheba, and saluted her with great civility; and when she asked him, whether he came to her as desiring her assistance in any thing or not, and bade him tell her if that were the case, for that she would cheerfully afford it him; he began to say, that she knew herself that the kingdom was his, both on account of his elder age, and of the disposition of the multitude, and that yet it was transferred to Solomon her son, according to the will of God. He also said that he was contented to be a servant under him, and was pleased with the present settlement; but he desired her to be a means of obtaining a favor from his brother to him, and to persuade him to bestow on him in marriage Abishag, who had indeed slept by his father, but, because his father was too old, he did not lie with her, and she was still a virgin. So Bathsheba promised him to afford him her assistance very earnestly, and to bring this marriage about, because the king would be willing to gratify him in such a thing, and because she would press it to him very earnestly. Accordingly he went away in hopes of succeeding in this match. So Solomon's mother went presently to her son, to speak to him about what she had promised, upon Adonijah's supplication to her. And when her son came forward to meet her, and embraced her, and when he had brought her into the house where his royal throne was set, he sat thereon, and bid them set another throne on the right hand for his mother. When Bathsheba was set down, she said, "O my son, grant me one request that I desire of thee, and do not any thing to me that is disagreeable or ungrateful, which thou wilt do if thou deniest me." And when Solomon bid her to lay her commands upon him, because it was agreeable to his duty to grant her every thing she should ask, and complained that she did not at first begin her discourse with a firm expectation of obtaining what she desired, but had some suspicion of a denial, she entreated him to grant that his brother Adonijah might marry Abishag.