1 Kings 1:13-15

Didst thou not, O king, swear unto thy handmaid? We do not read anywhere else of this oath: but, no doubt, David had solemnly sworn to her that he would make her son his successor, knowing that God himself had designed him to that honour. And it is probable that Adonijah was not an entire stranger to what God had declared to Nathan and David on this subject: and if so, his crime was the greater in setting himself to oppose the decree of heaven. Indeed he acknowledges as much, 1 Kings 2:15. The king was very old And therefore, probably, could not see so as to discern who had entered the chamber till Abishag, who ministered unto him, informed him.