2 Kings 9:6
And he arose, and went into the house; and he poured the oil on his head, and said unto him, Thus saith the LORD God of Israel, I have anointed thee king over the people of the LORD, even over Israel. And he arose, and went into the house; and he poured the oil on his head, and said unto him, Thus saith the LORD God of Israel, I have anointed thee king over the people of the LORD, even over Israel.
Acting at Elisha's direction, a messenger went into the house and poured oil upon Jehu's head to anoint him king over Israel. The anointing declared that Jehu's kingship was appointed by the Lord. It began a special commission from heaven against the house of Ahab.
That commission charged Jehu to destroy the house of Ahab. It was to avenge the blood of the Lord's prophets and servants shed through Jezebel.