In 2 Kings 22, King Josiah discovers the “Book of the Law” while repairing the temple:

2Ki 22:8 And Hilkiah the high priest said to Shaphan the secretary, “I have found the Book of the Law in the house of the LORD.” And Hilkiah gave the book to Shaphan, and he read it.
2Ki 22:9 And Shaphan the secretary came to the king, and reported to the king, “Your servants have emptied out the money that was found in the house and have delivered it into the hand of the workmen who have the oversight of the house of the LORD.”
2Ki 22:10 Then Shaphan the secretary told the king, “Hilkiah the priest has given me a book.” And Shaphan read it before the king.
2Ki 22:11 When the king heard the words of the Book of the Law, he tore his clothes.
2Ki 22:12 And the king commanded Hilkiah the priest, and Ahikam the son of Shaphan, and Achbor the son of Micaiah, and Shaphan the secretary, and Asaiah the king’s servant, saying,
2Ki 22:13 “Go, inquire of the LORD for me, and for the people, and for all Judah, concerning the words of this book that has been found. For great is the wrath of the LORD that is kindled against us, because our fathers have not obeyed the words of this book, to do according to all that is written concerning us.”

The question must be asked: what book did he discover? Christine Hayes answers in her Introduction to the Bible:

What was this scroll of the Torah that was “discovered” during Temple repairs. The scholarly consensus is that the scroll described in 2 Kgs 22 was Deuteronomy, or at least its legal core. First, the phrase “scroll of the Torah” appears once in the book of Deuteronomy but does not appear in Genesis through Numbers. Second, rural shrines and pillars in the worship of Yahweh are deemed to be legitimate in J and E; it is only Deuteronomy that contains instructions to destroy worship at local altars. Moreover, the story in 2 Kgs 22 describes the celebration of the Passover after the reforms are instituted. The celebration is not a family observance as depicted in older biblical sources, but a national pilgrimage festival celebrated by all the nation in Jerusalem, precisely as it is described in Deuteronomy. In short, there are several reasons to suppose that the scroll discovered by King Josiah corresponded in many ways to the oldest core of Deuteronomy.