In Moroni 7:5-6 we find: "For I remember the word of God, which saith by their works ye shall know them; for if their works be good, then they are good also. For behold, God hath said a man being evil cannot do that which is good . . ."
According to Monte Nyman, Mormon's quotation is very similar to 3 Nephi 14:16-20 or Matthew 7:14-20. Because these sources use "fruits" instead of "works" and "a tree" rather than "a man," it is possible that Mormon was paraphrasing these teachings. Another possibility is that he was quoting from an Old Testament source that is no longer in the Bible because of the loss of plain and precious parts. (1 Nephi 13:24-29) Further support for the latter possibility is that much of the so-called Sermon on the Mount was known to the Nephites before it was given as a collective sermon to the Nephites. (See Matthew 7:7 and 2 Nephi 32:4) [Monte S. Nyman, "Hope, Faith and Charity," in Studies in Scripture: Book of Mormon, Part 2, p. 302]