2 Chronicles 21:12

Overview - 2 Chronicles 21
Jehoram, succeeding Jehoshaphat, slays his brethren.
His wicked reign.
Edom and Libnah revolt.
12 The prophecy of Elijah against him in writing.
16 Philistines and Arabians oppress him.
18 His incurable disease, infamous death, and burial.
Treasury of Scripture Knowledge

2 Chronicles 21:12  (King James Version)
And there came a writing to him from Elijah the prophet, saying, Thus saith the LORD God of David thy father, Because thou hast not walked in the ways of Jehoshaphat thy father, nor in the ways of Asa king of Judah,

M. 3116. B.C. 888. a writing.
"Which was writ before his assumption." 2 Kings 2:1 ; Jeremiah 36:2 Jeremiah 36:23 Jeremiah 36:28-32 ; Ezekiel 2:9 Ezekiel 2:10 ; Daniel 5:5 Daniel 5:25-29

Elijah the prophet
If the account of the translation of Elijah be given in the order in which it happened, then it occurred in the reign of Jehoshaphat, the father of Jehoram. Hence, it is probable that he wrote it before his assumption, and left it to be delivered by Elisha or one of the prophets.
2 Kings 2:11

in the ways of Jehoshaphat
2 Chronicles 17:3 2 Chronicles 17:4 1 Kings 22:43

in the ways of Asa
14:2-5 1 Kings 15:11