1 Kings 22:41 KJV

And Jehoshaphat the son of Asa began to reign over Judah in the fourth year of Ahab king of Israel.

Jehoshaphat's good reign over JudahJehoshaphat's reign appears to have been one of the best, both as to piety and prosperity. He pleased God, and God blessed him.

2 Chronicles 20:31 And Jehoshaphat reigned over Judah: he was thirty and five years old when he began to reign, and he reigned twenty and five years in Jerusalem. And his mother's name was Azubah the daughter of Shilhi.
1 Kings 22:2 And it came to pass in the third year, that Jehoshaphat the king of Judah came down to the king of Israel.
1 Chronicles 3:10 And Solomon's son was Rehoboam, Abia his son, Asa his son, Jehoshaphat his son,
2 Chronicles 17:1 And Jehoshaphat his son reigned in his stead, and strengthened himself against Israel.

From OpenBible.info, most relevant first.

Timeline of the Bible918–889 BC · Jehoshaphat reigns in Judah; Ahab and Ahaziah reign in Israel

JehoshaphatJehovah-judged. (1.) One of David’s body-guard (1 Chr 11:43).
AsaPhysician, son of Abijah and grandson of Rehoboam, was the third king of Judah. He was zealous in maintaining the true worship of God,...
JudahPraise, the fourth son of Jacob by Leah. The name originated in Leah’s words of praise to the Lord on account of his birth: “Now will I...
AhabFather’s brother. (1.) The son of Omri, whom he succeeded as the seventh king of Israel. His history is recorded in 1 Kgs 16:1–34-22. His...
IsraelThe name conferred on Jacob after the great prayer-struggle at Peniel (Gen 32:28), because “as a prince he had power with God and...

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