Doctrine & Covenants 110:13-15

After this vision had closed, another great and glorious vision burst upon us; for Elijah the prophet, who was taken to heaven without tasting death, stood before us, and said: Behold, the time has fully come, which was spoken of by the mouth of Malachi—testifying that he [Elijah] should be sent, before the great and dreadful day of the Lord come---To turn the hearts of the fathers to the children, and the children to the fathers, lest the whole earth be smitten with a curse.