According to John Welch, the Nephites relied more heavily on the written law than the Jews in Jerusalem. The Nephites saw the law primarily as a written body (see 1 Nephi 4:15-16) and viewed any change in the written law with deep suspicion (see Mosiah 29:22-23). The Jews in Jerusalem in Jesus' day, on the other hand, had an extensive body of oral law to accompany the written Torah, and the oral law was very important in the pre-Talmudic period of Jewish legal history.
Accordingly, in the Sermon on the Mount, Jesus says repeatedly to the Jews, "Ye have heard that it was said . . ." (Matthew 5:21,27,33,38,43; italics added). To the Nephites, however, such a statement would not have carried as much weight as would a reference to the written law. Thus, in the Sermon at the Temple Jesus consistently cites the written law, saying, "Ye have heard that it hath been said by them of old time, and it is also written before you" (3 Nephi 12:21), "it is written by them of old time" (3 Nephi 12:27), "again it is written" (3 Nephi 12:33), "behold, it is written" (3 Nephi 12:38), "and behold it is written also" (3 Nephi 12:43). [John W. Welch, The Sermon at the Temple and the Sermon on the Mount, F.A.R.M.S., p. 99]