1 Kings 1:38
So Zadok the priest, and Nathan the prophet, and Benaiah the son of Jehoiada, and the Cherethites, and the Pelethites, went down, and caused Solomon to ride upon king David’s mule, and brought him to Gihon. So Zadok the priest, and Nathan the prophet, and Benaiah the son of Jehoiada, and the Cherethites, and the Pelethites, went down, and caused Solomon to ride upon king David’s mule, and brought him to Gihon.
Those named in the verse — Zadok the priest, Nathan the prophet, Benaiah the son of Jehoiada, and the Cherethites and Pelethites as the royal guard and David's loyal forces — were carrying out David's clear order and decision. They seated Solomon on David's own mule and brought him down to Gihon. The ride to Gihon was part of Solomon's public installation as king, leading to his anointing and proclamation as king there.
By promptly putting David's chosen successor in place, the action thwarted Adonijah's rebellion and kept it from gaining the advantage. The installation fulfilled the divine direction that Solomon should succeed David.