1 For behold, the day cometh that shall burn as an oven; and all the proud, yea, and all that do wickedly, shall be stubble; and the day that cometh shall burn them up, saith the Lord of Hosts, that it shall leave them neither root nor branch.
2 But unto you that fear my name, shall the Son of Righteousness arise with healing in his wings; and ye shall go forth and grow up as calves in the stall.
3 And ye shall tread down the wicked; for they shall be ashes under the soles of your feet in the day that I shall do this, saith the Lord of Hosts.
(3 Nephi 25:1-3, Malachi 4:1-3)
What did Malachi mean when he said that the “Sun of righteousness” (or in the Book of Mormon rendering, the “Son of Righteousness”) would “arise with healing in his wings?”
Shortly after the children of Israel were delivered from slavery in Egypt, they were invited to donate valuable possessions to build a tabernacle: gold, silver, brass, fine cloths, and precious stones. The tabernacle was a large tent which functioned as the center of worship for the Israelites during their years of wandering in the wilderness and for many years after their arrival in the promised land. The heart of the tabernacle was a room called the Holy of Holies. This room contained the Ark of the Covenant, a gold-plated chest containing the tablets of stone which contained the commandments given by God to Moses on Mount Sinai. The lid of the ark was known as the mercy seat. Two cherubs made of gold sat on it facing each other with their wings covering the seat (Exodus 25:1-21).

The Lord promised to meet with Moses “from above the mercy seat, from between the cherubims which are upon the ark of the testimony” (Exodus 25:22).
The Hebrew word for “atone” is כָּפַר (“kaphar”) which literally means “to cover.” (See Strong’s Concordance on biblehub.com.) The imagery of wings suggests protection and nurturing, which is desperately needed when a human being approaches the divine presence.
In the previous chapter, Malachi speaks of the purifying power of the Savior, who will “purify the sons of Levi, and purge them as gold and silver, that they may offer unto the Lord an offering in righteousness” (3 Nephi 24:3). In the passage above, he describes God’s presence as an oven, which burns away all impurities, leaving the wicked as “stubble” or “ashes.”
But, in the midst of this purifying power which purges all evil, the people who fear God’s name will be the recipients of Jesus Christ’s healing power. Just as the wings of the cherubs covered the ark and created a place where God could talk with Moses, the Atonement of Jesus Christ covers us and shields us from being destroyed by the purifying forces we will all experience. As a result, we can “grow up as calves in the stall,” with the protection and sustenance we need to achieve our full potential.
Today, I will be grateful for the covering and healing power of Jesus Christ, the power which both shields me from the destructive influences of the world and heals me as the impurities in my own soul are purged. I will be grateful for the “heav’n-born Prince of Peace,” the “Son of Righteousness,” who brings light and life to all and who is “ris’n with healing in his wings” (“Hark! The Herald Angels Sing,” Hymns, 209).
Does it bother you that Nephi used an idiom found nowhere else in the scripture except Malachi’s usage hundreds of years later?
It actually used to bother me, but it doesn’t any more.
There are many passages in the Book of Mormon which mirror passages in the Bible. Some are easily explainable, like Nephi quoting from the book of Isaiah, which was on the brass plates. Others are likely attributable to a common source which we don’t have, like the Allegory of the Olive Tree in 1 Nephi 10 and Jacob 5, which also appears in Romans 11. Many shorter phrases probably represent Joseph Smith simply using King James English to convey the meaning of the original author.
But there are some passages which are more difficult to explain. Consider Paul’s discussion of charity in 1 Corinthians 13:4-7 which also appears in Moroni 7:45. Or Paul’s list of spiritual gifts in 1 Corinthians 12:8-10 which is nearly identical to Moroni’s list in Moroni 10:9-16. Or the “Faith Hall of Fame” in Hebrews 11, which has a similar structure to Ether 12:11-22. I think the most likely explanation for these examples is that God can reveal the same thing to multiple people.
Nephi actually tells us this is true. At the end of his vision in 1 Nephi 11-14, he tells us that John the Revelator would see many of the same scenes from his vision, centuries later (1 Nephi 14:18-27). So I don’t find it hard to believe that Nephi could use phrases which Malachi would also use about a hundred years later. (Besides 2 Nephi 25:13, you will also find hints of Malachi in 1 Nephi 22:23-24 and 2 Nephi 26:4-9.)
A much longer answer than you anticipated, I’m guessing! The main message is this: I believe that both Nephi and Malachi were messengers from God, and I’m comfortable with the idea that they could use similar or even identical language to convey messages they received from Him.
I hope that’s helpful. Thanks for the question!