Charles Ellicott Commentary 1 Kings 1:13

"Go and get thee in unto king David, and say unto him, Didst not thou, my lord, O king, swear unto thy handmaid, saying, Assuredly Solomon thy son shall reign after me, and he shall sit upon my throne? why then doth Adonijah reign?" — 1 Kings 1:13 (ASV)

Did you not ... swear. —We have no mention of this oath elsewhere. It may have belonged to the time of Solomon’s birth (2 Samuel 12:24–25). In 1 Chronicles 22:6–13, we find a designation of Solomon for succession, apparently earlier than this time—it being clearly understood (see 1 Kings 1:20), according to Eastern custom, that such designation, without strict regard to priority of birth, lay in the prerogative of the reigning king.