According to John Welch, the New Testament phrase “Thou shalt love thy neighbor as thyself” (Matthew 5:43-44; compare 3 Nephi 12:43) was not a phrase invented by Jesus in response to questioning Pharisees. He is quoting there from the book of Leviticus (Leviticus 19:18)… .
D&C 104:18 defines the law of the gospel as follows (this is the only place I know of in scripture where this term is defined): “Therefore, if any man shall take of the abundance which I have made, and impart not his portion, according to the law of my gospel, unto the poor and the needy, he shall, with the wicked, lift up his eyes in hell, being in torment.” What we have then, in this part of the sermon can, I think, aptly be described as the law of love, the law of the gospel. “Love your enemies, bless them that curse you, do good to them that hate you, and pray for them who despitefully use you and persecute you; That ye may be the children of your Father who is in heaven … ” (3 Nephi 12:44-45). [John W. Welch, “The Beatitudes--Christ’s Teachings,” in Teachings of the Book of Mormon, Semester 4, pp. 140-141]