Flavius Josephus, Antiquities of the Jews
William Whiston, A.M., Ed.

[140] Now when Athaliah, the daughter of Ahab, heard of the death of her brother Joram, and of her son Ahaziah, and of the royal family, she endeavored that none of the house of David might be left alive, but that the whole family might be exterminated, that no king might arise out of it afterward; and, as she thought, she had actually done it; but one of Ahaziah's sons was preserved, who escaped death after the manner following: Ahaziah had a sister by the same father, whose name was Jehosheba, and she was married to the high priest Jehoiada. She went into the king's palace, and found Jehoash, for that was the little child's name, who was not above a year old, among those that were slain, but concealed with his nurse; so she took him with her into a secret bed-chamber, and shut him up there, and she and her husband Jehoiada brought him up privately in the temple six years, during which time Athaliah reigned over Jerusalem and the two tribes.