Book of Malachi 3,1-4.23-24.

Thus says the Lord God: Lo, I am sending my messenger to prepare the way before me; and suddenly there will come to the temple the Lord whom you seek, and the messenger of the covenant whom you desire. Yes, he is coming, says the Lord of hosts.
But who will endure the day of his coming? And who can stand when he appears? For he is like the refiner's fire, or like the fuller's lye.
He will sit refining and purifying (silver), and he will purify the sons of Levi, Refining them like gold or like silver that they may offer due sacrifice to the LORD.
Then the sacrifice of Judah and Jerusalem will please the LORD, as in days of old, as in years gone by.
Lo, I will send you Elijah, the prophet, Before the day of the LORD comes, the great and terrible day,
To turn the hearts of the fathers to their children, and the hearts of the children to their fathers, Lest I come and strike the land with doom.